There's a post elsewhere in this forum (I think) where Scott M. does a good job of explaining it...but basically, the portables aren't designed for the different tension of 90-meter tapes, so your heads are worn out quicker because 90's aren't re-tensioned.
I've mastered on 90's quite a bit and have never had a problem in six years; never even had to service my deck once.
As for the Sonys, the best price I've seen is from American Digital (am-dig.com). The DGP60's go for $2.19 for a 10-pack, $2.16 for a 30-pack, and $2.09 for a 50-pack. The DGP90's go for $2.29/$2.26/$2.19 for the same packs. Decent prices on the Maxells, too, though I haven't shopped around too much for those.
--Dave