What are the pros and cons of having a rifle or midlength gas system on an 18” barrel AR-10? I don’t know what one to get for my build.

Going with a mid-length on an 18" would, IMHO, be akin to a carbine-length on a 16" and a rifle-length would be about like a mid-length on a 14.5".

Now beware, I did not do the math, so this is not exact.

Definitely go with rifle length, it will be a smoother shooter. Recoil is slightly less which enables you to keep your scope on target during shooting. Really there is no way I would have a midlength gas system on on a 18 incher or even a 16 incher (and I don't).  Now when you go less than that it's a different story.

I run a rifle length on my 16" .308. It was a little harder to tune, but recoil seems to be milder compared to the 16" mid length it replaced

Personally, I'm going with rifle-gas on my 16" and intermediate on my 14.5", if Randall ever gets my barrels back to me.

5.56mm works great with rifle gas and 18" barrels. Other calibers not so much. .308 should be fine.

The only problems I ever had with rifle gassed uppers was 7.62x39mm and 6.8SPC. Opening the gas port fixed them.

I went rifle length gas  with my 18" Criterion Hybrid , SS, .308 barrel.

Seems to function well.  Not sure if it's true or not but I figured with.308 the longer gas system may run smoother.  

Thanks guys for all your input, I think I’m leaning towards a rifle length gas system now.

Let me know how it works out!  I have been looking into this as well.  I will be going with an 18" barrel.  A few sites suggested mid length gas tubes but it seems to me that rifle length would work and seems to be a better option if it does.

As a general rule, I like going with the longest gas system that will reliably function.  With the large frame guns, I wouldn't want a midlength gas system for anything longer than 16".  For example, my 18" 6.5 Creedmoor has an extended Rifle +1" gas system, and it runs like a dream.

On the AR15 platform, I've been gradually upgrading my cheap barrels and through the process I am also moving to longer gas systems (16" Carbine to 14.5" Mid-length; 18" Mid-length for 18" Rifle) and am pleased with the results.  For what it's worth, I think the quality of the barrel is more important than the gas system location - but all else being equal, a longer gas system would be my preference in most cases.