So as long as you can get by without any additional outputs, you'll be all set with the Montego A3DXstream. The gameplay performance increase is not really anything noticable in an actual gameplay situation. It is however noticeable during benchmarking. Of course you're still only seeing a 1-4fps difference depending on your system. Not really anything to consider in the decision making process in my opinion.
Sound quality is another issue.
The improvement in range of sounds from the AWE-64 to the Montego A3DXstream was, at least for myself, a big improvement. Sounds that I never even knew were there using the AWE-64 were suddenly crystal clear. Those light tinkling sounds and low rumblings in Unreal were all to evident now. I also noticed none of those speaker pops and hissing that so often accompany some games. The game literally took on a whole new feel with this added depth of sounds.