they are basically the same watch, with one being a GMT and one having the power reserve function (as well as cosmetic dial differences). also, based on the pix from panerai's Web site, it looks like the 88 has the quick-change strap system and the 90 does not (you can't fully go on the pix, though, as they can be out of date). i have the quick-change system on my 164 and think it's GREAT.
i personally MUCH prefer the 88 over the 90, as i like the hands and silver sub-dial of the 88 quite a bit, and the power reserve indicator on the 90 doesn't do much for me. i also prefer the four lines (two above, two below) on the 88's dial over the three lines (all above) of the 90's.
however, you do have a GMT already, so, maybe you want to diversify?
good luck with the hunt!
