There are some good points here.
The 122 is certainly appreciating at a faster rate, and a decent example will lose no value if kept in good shape. I think the same is true for the MGB, but it's not appreciating at the same rate (at all?) Value will remain stable though, I'd imagine.
Space isn't really a concern. This isn't a 'put people in the back' kind of of toy. It's for me and my girlfriend to cruise in, maybe take some camping trips. Storage is adequate in both for my needs.
Parts availability is far better for the MGB, but the 122 has some decent support these days, too. Agreed on rarity of parts outside mechanicals.
Swapping engines isn't in the plans unless needed, for either car. I'd simply run it with the B18 and enjoy things as-is. I reserve the right to completely pancake on this statement... Suspension, wheels, tires, etc. are absolutely items of interest.
Fit-wise, I'm 5'11" and 160, so nothing special to see here. Fit is fine for both, 122 of course requires less neck tweaking at lights.
They're both aesthetically pleasing in different ways. The 122 is just the prettier car, hands down. It has a Pininfarina quality to it. The MGB is a lump in comparison, but a lump with shooting-brake proportions and I love that. Agreed on the 122 being the rarer, more interesting car.
The two specific cars that I'm looking at are both imperfect. Both have been repainted, with the 122 showing some bubbling and shoddy body work in areas. That scares me, and I realize that I'd have to mitigate that in order to retain it's value (or would I at $6500?...).
I suspect that the Volvo will be the more reliable pile, and while this doesn't matter all that much, I certainly want to minimize the amount of times I'm left stranded on the road. An MGB purchase might warrant a new main harness just for peace-of-mind. Can past owners here comment on this aspect?