yeah, 6 10's just means you have to cut out alot more holes, wire alot more speakers, etc. Just a pain. 2 15's is going to have very similar performance with alot less work involved.
You probably won't see a huge gain without upping either cone area or power. All things equal there isn't much difference between the 21 and 2 15s. I just made the switch to a 21 and in my case the 21 is the more impressive setup but things weren't equal.. if I had to guess in the same car similar box it'd be close. How much airspace do you have