Hi. The 350 will give you more velocity with the same amount of air so if you don't mind the length of the 350 go for it. It folds almost in half in the 350 variant. I have a .30 lothar barrel and what I believe to be an older style flanged 350 barrel in. 30 also for sale if you are interested PM me. Good luck. I find myself using the 250 and 450 the most. The 250 I have the behomouth so if I detatch the stock it can fit anything the butt stock can. But I also have my 450 set up to fit in most backpacks.the 350 a great middle ground if I didn't have 2 leshiys I would consider just sticking with the 350 for the best of both worlds.
Additionally... a 350mm L2 can fold just a compact as the 250mm version if you ditch the factory shroud and replace it with a short handguard and a detachable moderator of your choosing. There are many options out there. The Behemoth would be the fastest to attach and remove.
OP what did you end up choosin,? I'm between 350 and 450 at the moment. Going to see Brian at Edgun this week. Would appreciate your thoughts on what you chose