Here's my take on 300 vs. 310 VCOs.
Find a way to try both. Depending on the room you have in your system
to devote to VCOs, I'd start with one or more 300s. The extra
triangle and sine outputs are invaluable, not to mention linear FM
and sync inputs. You really have a much wider tonal palette to
synthesize from.
Once you have a couple of 300s, the 310s are great for getting a lot
of bang for the panel space buck. They have a nifty variable saw to
pulse output that the 300 doesn't.
Moe
http://www.hotrodmotm.com