I own a 650 and bought it after trying a 400, thinking its newer technology would compensate for its smaller displacement. For 80/20 type riding that wasn't the case. Neither bike is an interstate cruiser, and neither is a great single-track dirt bike. From an overall perspective, either bike is fine.
By the way, if your dealer actually suggested a DRZ650, find another dealer, or at least another sales rep. I am actually sympathetic with sales people who have to know a complex and changing product line, but the DR and the DRZ have been in Suzuki's lineup unchanged for 15+ and 10+ years respectively.