Price-wise, in today's market, I'd say the two rifles are comparable and make good swap for both involved.
If I could only have one, I'd take the PTR-91. These things are rock-solid battle rifles that shoot a full-power .308. IMHO, there are almost as good as the HK-91, and by swapping out some of the internal parts with real HK parts, they can be made to be as good. During the ban, it was HK-91's were in high demand and you were lucky to find one for less than $2,800 - 10+ years ago.
True, ammo costs more, but .223 ammo is ridiculously priced as well. I would put the collapsible stock on it and the PTR-91 would be my "go-to" rifle. Then, I'd just buy a .22LR for practice.