Unless you're going to be living in the DSM web portal then you won't really notice too much as all the other interfaces will be the same. Of course I'll move to DSM 7 when it's ready but for now it's DSM 6.
If you really want a taste of DSM 7 on your new DS1520+ then you can install virtual DSM 7 RC in Virtual Machine Manager ... for which you get one free vDSM licence.
Oh yeah, photo management ... DSM 6 has the choice of Photo Station and Moments, but these are replaced in DSM 7 by Synology Photos. So there'll be work to change that. That's probably one of the biggies to consider.