I own both as well and I do like both at the same time. The 650 is the more relaxed and fun one, while imo the DT990/600 has the even better soundstage, separation and the more analytical sound.
Bass is good but different on both (deeper on 990s?), and then there's the Beyer treble... I think they're both great and complementary
You can read all about their soundsignatures in Zombie_x's great topic: http://www.head-fi.org/forum/thread/513393/guide-sonic-differences-between-dt770-dt990-models-more
Unfortunately he's only comparing it to the HD600, but still might help.