The downside to the 433 is that they only seem to have 3 cabin versions. Also, only 2 heads. Not what I'd consider ideal for 8 people. The living space though would give you plenty of space. Lots of space, inside and out so aside from maybe too many cooks in the galley I don't think it would feel crowded until everyone went to sleep. You'd be using the main living space as a sleeping space and I'd think that would make it feel more crowded. Best bet would be to discuss with the other couple.

514 with 4 cabins, 4 heads, plus a v-berth and crew cabin would be ideal giving everyone a space to sleep.

If it was me, I'd go with the 514. Try to get owners time to cut the cost - that should make it closer to the cost of the 433.