This is a bit of a tough one because normally I always recommend 4' cages for adult ball pythons, but I do not really love wood enclosures. They hold heat, but unless they are well finished on the inside, they are difficult to clean and porous. These are also used, which will make them difficult to sterilize from their last inhabitants.

Also, as you state, people do keep all sizes of adult ball pythons in CB-70 style tubs (34" L x 18" W x 6" H) for their entire lives. However, if your BPs grow large enough to need "X-large" size hides (usually around 2400g+), you will find this is not really enough room for two hides and a water dish. In a rack system you don't need the same hide setup, so you gain some extra floor space, but in display cages, I really feel 4" is the ideal. Not too small and not too large. Granted, some extra width on a 3' enclosure can help out quite a bit.

Depending on the construction of the enclosures -

For now: Either plan is better than having your two ball pythons housed together. I am never a fan of this practice for multiple reasons.

In the future: If you trust the guy and think you can clean out the 4' wood cages well then both ball pythons could be housed in these for their entirely lifetimes without issues. I do not know the age of your snakes, but as previously stated, 30" may be cutting it close if they end up on the larger side.

If it were me, I honestly probably wouldn't take either. I would go buy 2 IRIS CB-70 tubs for like $20-$30 for now, then save up the $300 for two AP T-8 cages (better yet T-10s).