They're both good amps. I'd actually lean to Arni's side on this one, since the SA250 is a bit sweeter in the highs. Then again, the SA250 doesn't have anywhere near the overall grip that the 990 does. If you can, Kurtisk had a great suggestion--buy both and see which one you like more. You can almost certainly sell the one you don't keep for what you'd buy it for...
Best,
Bob
Best,
Bob