We'll guy I gotta say it depends on preference, some like single point slings and some like two point slings, and some even like three point slings, but since the question is which between single and two point slings, I'll answer that question the best I can... As for the reason to use a single point slng, it gives you a wider range of motion and control in standing, kneeling, or prone positions, so it gives you a larger range of movement, control and maneuverability of the weapon when firing or moving and stuff like that, but a two point slng is better for carrying the weapon when not firing because the two attachment points hold the weapon more stable and against the body and doesn't swing around knocking things over or banging against your legs or sides and stuff like that so for more stability when carrying the rifle it's better to have more solid control of the weapon but for shooting and changing from strong side to weak or off side you'll want a single point sling for this type situation. With the MS sling series you have both scenarios covered no matter what your specific need is at any given moment or situation. Brownelles has a YouTube video showing the MS1-MS4 sling series which shows you the versatility of the sling system, one of the biggest differences between the MS1 and MS3/4 slings is that the MS1 uses a sliding attachment on the body of the sling and the MS3/4 slings have a loop on the sling body that you can hook with your thumb to adjust length without missing a step so you can tighten or loosen the slng if you need more or less slack or need it tighter agains the body, and it works equally well in either one point or two point configurations, the MS1 also adjusts just as easily as the loop setup but you don't have the big loop sticking out, but each have their PRO's and CON's so again it depends on what you like and what you need but either way you can pretty much fill any need with the Magpul MS sling series. If money is a problem then order the less costly MS1 and then next payday order the attachments you need, the standard MS1 runs between $25.00 and $30.00 on average, the attachments range from $10.00 to $20.00 give or take a few bucks and can be purchased from several different websites from Amazon to brownelles and my favorite Valhalla Tactical supply so if you haven't already purchased either a one or two point sling then kill two birds with one sling and you'll be very happy with the wide range of options available to you
The only way to tell which you'll like best or which serves your needs best is to try each type, however if your unsure and instead of buying two seperate slings why not just buy one that fills both roles? I've tried about every type sling 1, 2, 3 point slings but it's a toss up between the single and two point slings because typically one sling doesn't fill both roles but with the Magpul MS series slings they do, as of now they have the MS1, MS2, MS3, and MS4 sling systems with several configuration options for each model. So what I did was order a Magpul MS3 series sling which can be configured to either one or two point configurations, There is also the question of what your weapon is set up for? Nowadays most stocks and receiver end plates and fore ends can work with QD=Quick Detach or single point loop or Ambi loop on the receiver end plate, Magpul offers several versions and several adapter options, I first ordered the MS3 with paraclips, one end with the paraclip attaches to my Ambi receiver end plate on either side and the paraclip on the other end can attach to a ring above the first paraclip for single point option, or to a forward sling loop for a two point option and if you don't like the paraclip type they make a single QD and paraclip, or dual QD or a combination of the two so that solves the question of either or because you have both. I also ordered the original MS1 sling which does not come with any attachments, it's made to use with military type front and rear sling loops, however you can order several different attachments for whatever you like or need, so I ordered one of each type attachments so I can swap things around to fit any need at any time and since I have more then one AR rifle I have two very versatile sling systems that work for any configuration I need at any time.. Magpul also makes a few mounts to go with their slings, they offer two rail mounted options that mount to your picatinny railed Handguards, they make a rail mounted sling loop for the paraclip type adapter and a rail mounted QD mount which is extremely nice because your not confined to the standard front sight sling loop and you can place these mounts anywhere on your railed fore end you like, top, bottom, left, right, the front or rear or middle on your Handguards so it's nice to be able to place the mounts where they work best for you specifically. The newest AR I built has a 16" inch carbine barrel and I used a Midwest Industries T9 monolithic quad rail which is just over 9" inches long which means I don't have a front sight tower with sling loop, and I wanted it this way so I could use an extended quad rail instead of the standard 7" inch carbine set and I used a low profile gas block so the QD rail mount was the perfect option for me and the new rifle and I can mount any configuration of the MS slings any where and any way I want to., yes Magpul can be a little more expensive then other manufacturers but for the quality and versatility they are one of the best on the market and have items for almost any budget and to fit any need.
So if you can't decide which then start with the MS1 sling and order the seperate attachments to fit whatever your preference or weapon configuration will allow and then customize it from there. I am using the MS1 with dual QD adapters but can change either or both ends whenever I need to, they also come in Black, OD Green, and FDE or Coyote Tan and the adapters and connectors also come in matching colors, since the general theme of my newest AR build is black and FDE/Tan I went with a matching color for the sling..
One last thing, even though Magpul can be a little more expensive then some brands, there are much more expensive brands that stole parts of magpuls designs and are selling similar slings for two and three times the price of magpuls setups but they do not offer all the adapters and fittings and mounts as Magpul does, if you buy a cheaper similar sling then be careful because a lot are made to handle only soft air rifles or AEG type play toys, I'm a firm beleiver in you get what you pay for and if something is a copy of a name brand and charging much more then your getting screwed, and if you buy a chaper knockoff then your asking for the product to fail, also be cautious of anything that says Magpul PTS, because even though it says Magpul ITS NOT MADE BY THEM!!!!! These items are licensed cheaper made knockoffs so stay away from anything Magpul that has PTS in the description because some less then honest sellers will try to pass off these items as being made and sold by Magpul and saying they are of the same quality when they are not
Anyway I hope this information will help you decide what you need, or at least explain a little bit of what each type sling is used for? So good luck and have a great day