To me, there are really two different philosophies in the interior of the C30, C310, C315 & C320: more room in the main salon or more room in the forward birth. It comes down to where do spend your time.
The C30, C315 and C320 all have traditional v-births. Meaning, they are tight, foot first spaces that aren't particularly comfortable. But the compromise is the large salon. There are two long settees that could be births. The space around the table can fit more people. You stairs are steeper and the engine has been shoved into the smallest space possible with tough access for doing more extensive repairs and maintenance.
The C310 is a completely different concept in regards to the v-birth and salon. The salon is smaller and less preference was given to this area. One settee is long and can be used as a birth (although it is a bit narrow) and the other is short and only really good for sitting. You can't fit very many people around the table but two are really comfortable. The v-birth is a queen bed. You get into it like a regular bed and it's an innerspring mattress. It has stairs to get below that are not very steep at all. This allows for much better access to the engine. (with a simple upgrade you have 360 access, tell me there's another 30 footer with that)
It comes down to how you will use the boat, IMO. If you mainly plan to day sail, the C30, C315 and C320 is the better set up. If you plan to cruise, especially as a couple, than the C310 is better.
The double sink vs. single sink is a tough one. We have a single and have often thought of going double. But that would reduce the counter area. I think a double with two filler sections, one for each sink, would probably be the best way to go.
