In line with what Grado says, the SR80is drive more bass than the original model, but the trade-off seems to be less sweetness in the treble response. The fit is also less comfortable with the new pads, though you can try buying the original pads as a spare part. I experimented with swapping the old earpads and the new between headphones, while the new pads on the original SR80 sounded worst, both sets sound great on the new SR80i. If you want to keep the newfound warmth of the SR80i leave them as is, but for $20 the "L-Cush" are worth experimenting with for added zing and comfort.
Despite the low-end improvements, the SR80i model is still less bassy than the SR60, with the scale tipped the other way in favor of treble detail and immediacy. If you're looking for something a little more even with both treble detail and depth-charged bass, skip over Grado entirely and opt for the closed headphones, which are very impressive for the extra money. What they miss, however, is the Grado's light touch with music.
I am a big fan of thein-ear headphones, and the SR80s actually sound more like them than they do their own kin. Think of the SR80is as the "sitting at home" version of the UE's travel-friendly design.
If you're looking to spend less than $100 on a pair of headphones, my money would go to the slightly more detailed and involving SR80is than the cheaper SR60is. The small trade-off in bass response is worth it for the extra layer of detail.
On the other hand, if you own the original SR80 headphones, there is no compelling reason to get the SR80i update apart from the slightly warmer sound.