I'm thinking of putting together an AR, but I'm curious as to the right gas length for an 18 inch barrel. I've heard intermediate is the best, but as most piston companies and barrels don't offer intermediate. This leaves mid-length and rifle length. I know either can be adjusted to make sure the bolt isn't over-gassed, but what about on the gasblock itself. Does that hold up better to the reduced pressure of the rifle length system? However, the mid-length gas setup is a bit lighter, and I'm going for lightness, and I don't plan on using a bipod. Which is better?

Rifle length DI is reliable and soft shooting - it is the gold standard in 3-Gun competition. Not so sure about piston as I never had much use for one.

Yeah, I've heard the rifle setup is really soft, and it'll be easier on the gas system itself I'm thinking. I know an adjustable mid-length gas system could ensure the bolt isn't damaged, but I was wondering about the possibility of increased wear on the gas system itself. I know a few inches down the pressure is much less.

As long as possible; in your case:  rifle!

Rifle.

This right here.  99% of what I run now are 18" with rifle gas.

I built a rifle DI 18 and it is sweet.

But I got two middies DI18 that is also sweet to shoot to.

Rifle length is the correct length for 16-20" in my opinion.

18" and greater.   A 16" won't run reliably with a rifle length gas system in most cases.  

It's generally better to choose the longest gas system you can.  

It depends on your piston system if they have a requirement. For Adams Arms piston I am running an 18" mid-length gas that runs perfectly fine. When I first built my 18" I did ask the same question like yourself and it was a toss up mid vs rifle length. Another thing to consider if your gas block is fully adjustable that'll also give you room to control the amount of pressure/recoil.

best barrel length for 5.56 and game guns is
Car 10.5 - 11.5
Middy 14.5
Int 16
Rifle 18

Lengths less than that get extra picky about ammo, suppressors, etc.  Longer than that means more dwell so less picky, but longer barrel.

308 follows a pressure curve VERY similar to 5.56 albeit with more gas available.  Most 16-18" 308's have a MIDDY system which is just fine.  My 18" 308 has a RIFLE system and I use it exclusively for 3 gun.  It kicks like my 6.8.

Since you are using a piston system, you can get by on less dwell.  I have a seen a 14.5" RIFLE that used a piston and it ran fine.  After all, the "cylinder" is on top of the barrel so its got all sorts of gas available.  I just ordered a SA piston system for my 12.5 MIDDY SBR (very shot dwell) because it is just too picky as a DI gun.

For you I vote RIFLE for performance, but if you are trying to shave 1/2 oz on the operation rod, then go MIDDY for less weight.

I have to agree with most of the posts.
 I have all three sizes.  My 16" piston came with a carbine gas port, and for an AR it has some recoil.  My 18" DI has a mid size tube, and it is sweet (kind of wish my 16" had a mid size).  And my 20" DI has a full size tube and has next to no recoil (but it has a lot of extra weight to eat up any recoil I guess).  
In the end I think the mid size is great for an 18" (I have never shot an 18" with a full length tube).
As far as I know, any piston system on the market will demand the size of gas system (port position), so you will not have a choice once you have the piston system you want picked out.
Now again, as far as I know, I did not think anyone was making a rifle size piston system, but I don't know for sure.  It would have a very long opp rod.

I can only add that I am still happy with my piston system.  I have a lot of rounds down it, and it just works great.  

AS