I think you would prefer the DR650 for what your looking to do. You may want to add a oil cooler aux cooling fan for when your stuck in traffic or riding slow in high heat for an extra piece of mind, is it needed? probably not.
I went with the DR and feel as if I made the right choice between the two. For mods I did the airbox mod, twin air filter and needle swap and the bike deffinetly runs stronger, Im still saving for a procycle SS1 or Leo Vince exhaust. If I was heavier and did more off road I would address the suspension sooner. If I rode more in the the southwest I would consider an oil cooler fan for piece of mind, knowing many have used these bikes in hot conditions without. I prefer the simplicity and torquey nature of Dr650.
Another thing to consider is the gearing of both bikes, I have read where the Drz400 gets buzzy over 65mph.