I'm a beginner, too, in the AR15 world. My likes have evolved. My first thought was to get an AR15 that had a 20" barrel and a fixed carry handle rear sight, and maybe even a "heavy barrel" (called an HBAR - it's as thick beneath the forearm as it is outside it, whereas non_HBARs get thinner beneath the forearm to reduce weight). I'm old enough to remember Vietnam (but young enough to not have to have gone) and so to me the AR15 with a fixed carry handle and the 20" barrel and the plain old standard buttstock represented the AR15 to me more than the modern versions with shorter barrels, removable carry handles, collapsible stocks, and now even fancy forends for "tactical" stuff like flashlights and vertical forearm grips - even though in Vietnam the rifles didn't have heavy barrels, and there were other differences from what is now called the AR15-A2 style.
Two things changed my mind. Well, three, I guess. (1) my girlfriend and I got to shoot, for just the cost of ammo, an AR15 with a 16" barrel and collapsible stock, (2) my girlfriend, who is new to firearms, loved shooting the AR15, and that collapsible stock was better for her (and so for us) because of the variable length of pull it allows, and (3) I started reading at ar15.com and looking at the pix here, and learning the basics (because I knew nothing whatsoever about this firearm) and then I finally set up a profile so I can do basic searchs over the last 30 days. I got to like the 16" M4 style guns - the M4gerys, as they are called here; and I planned to get one myself.
A lot of people on this site talk about "building" an AR15. There are two reasons, I think, that people like to build their own: they want to save money, and they want to have complete control over what components are in their AR15. But saving money and "having the best components" could be mutually exclusive, so everyone who's going to build an AR15 is chosing somewhere between the two extremes of top quality components and "good enough" components. You have to decide, too, where on that possibly confusing scale of cost, value and quality you want to be with your first AR15.
I had every intention before yesterday of getting a Bushmaster 16" M4 style "Patrolman's Carbine". The M4 style barrel has that step down area that's really there because in the Military version of the M4 that step down area accommodates a grenade launcher. It's a very popular style of AR15 now. Sort of a SWAT concept or CQB (close quarter battle) concept carbine. The short barrel makes it easier to move around in confined areas (hence SWAT and CQB) or to get in and out of a vehicle. You can imagine, too, how that's appealing to anyone who's concerned about needing a firearm during civil unrest. The shorter barrel makes the gun easier to move around with, and imagining needing this in an urban area brings to the imagination the idea of running crouched with your several loaded mags and your short carbine, engaging in close range firefights. Your buddy with the .308 will of course be hidden in some secure area picking off selected dangers and not need a short barrel or a lightweight firearm.
I said that even a few days ago I was planning to get a Bushmaster Patrolman's Carbine, and actually this is one I can recommend to you. The only reason I'm not going to get one myself is because yesterday I was reading in the Equipment Exchange on this site and discovered that a dealer was offering a 16" barrelled complete upper that appealed to me more than what that Bushmaster would have offered. The barrel profile on this upper is different than the M4 style barrel on the Bushmaster (it's not a step down and it's not an HBAR either, although the M4 style also isn't an HBAR) and there are two other differences: the twist rate is 1/7 instead of 1/9 like the Bushmaster has, and this upper has a mid-length gas system, which changes where the front sight is on the barrel: it puts the sight a bit further towards the muzzle. I ordered this upper today. So now I have to buy a lower separatly from this upper and I'm going to eventually "assemble" my first AR15, meaning I'm going to put two pins into an upper/lower combo that I will have bought unjoined. It's possible I might "build" my lower, but right now I plan to buy one completed by the vendor. This AR15 I'm in the process of aquring is going to cost a little bit more than that Patrolma's Carbine, I think, but I'll be happy with it and I don't mind waiting a little while to get what I'd rather have.
My feeling on firearms, and sometimes on other things too, is to chose "quality over quantity." Why have a lot of second rate stuff when, if you're willing to have less stuff, you can have only the first rate? It does come down to money for a lot of us, and we always have to find the right balance between what we want and what we can afford.
GL