At the prices KEH.com is selling them for--get both! That's what I did.
Mine are also both razor sharp. Shortly after getting the 90 (I got the 127 first), I took a full-length portrait of a friend with it, leaving a fair bit of room beneath his feet and above his head. On the negative (FP4+ in Rodinal) I could clearly and sharply read the lettering on the button holding his jeans up.
I don't know exactly how much these lenses resolve, but at 12x16 print sizes they render superb detail, and it is obvious there's even more there that would be revealed at larger sizes.
As I use mine in the studio, and as my space is a bit small, I do find them both useful despite the relatively minor difference in focal length. I use the 127 most often for mid-body and upper-body portraits. The 90 is just right to do full-lengths in the space I have available.
If you decided on only one, I'd probably vote for the 127. I think it could do what the 90 does--if you can back up a bit--but won't give quite as much wide-angle exaggeration as the 90 if you get up close to people. And it is physically smaller, so you can tote it around a little easier. Otherwise, they're both fantastic in my opinion.