I think there are a few aspects to this ......
(1) What are your needs? will either the 31 or 24 give you what you need in terms of space - only you can decide that!)
(2) Big difference in $$? how big and how meaningful to you?
You may well fid that the one that "Just need the AC" may in fact need a whole lot more! If you are not familiar, it would be good to have someone look at it for you if you are leaning to that one! If it needs a lot of work, and unless you could do it yourself, could be a real money pit!
(3)Some RV's advertised as totally restored are not that at all! Again, you need someone knowlegeable to check it out. There are people on this forum that maylive near you willing to help.
On the other hand, If it is well restored and is being sold at a good price because the vendor needs $$, it could be a bargain.
I have spent a lot of $$ in years gone by restoring British sports cars and never been able to cover my expenses on selling them and you could find a situation like that if you are lucky with a restored Airstream - or you could be spending way too much on something that has not been properly done at all. Again, best to get someone to look at it if you cannot judge.
Just so many variables, I don't think you can get a simple answer!
Brian & Connie Mitchell
2005 Classic 30'
Hensley Arrow / Centramatics
2008 GMC Sierra SLT 2500HD,4x4,Crew Cab, Diesel, Leer cap.