The single crossover of the AVP7 offers the same settings as the 990 as far as the frequency specified for the crossover point, but the same crossover is applied to all small speakers (mains, center, surrounds). With the 990, you can "dial in" your bass management so your mains are crossed over at one point, center at a different point, side surrounds at a different point, and rear surrounds at yet another different point. How valuable this can be depends on your speakers.
Doug917 (who posted in the two threads linked in sluggo's post) has heard both. He preferred the 990 in two-channel, but felt that the AVP7 might have a slight edge in movie playback.
There is certainly an opportunity to keep some money in your pocket by going with the AVP7, and I suspect the sound would be very close to the 990's. It makes sense that the Sherwood software could provide an avenue for firmware updates, although I'd check with the warranty provider to see if loading the Sherwood firmware will void the extended warranty (it shouldn't hurt anything, but it might be an excuse to turn down unrelated repairs). My suspicion (based on experience with Outlaw over the years) is that there's more customer service muscle and future resale value tied to the 990. Whether that's worth $290 is a decision I can't make for you, but you'll want to make that decision fairly soon - from what I've seen online, the AVP7 won't be around much longer. I'll also say that I've got no buyer's remorse about my 990, for what that's worth.