From what I've heard, even the "new" rafts with ballast bags are designed expecting a human ballast of a specific size. Two guys in a ten-man raft are going to be tossed around. In a "crowded" four man raft, they are more likely to be able to wedge themselves in place and get tossed a whole lot less. And, the raft will be riding lower & more stable in the water
So...not having spent any quality time camping out in life rafts...from all I've read or been told, try to match the crew size to the raft size. I'd also vote for two 4-man rafts over one 8-man, simply because there are too many nagging questions and accusations about raft failures. There's also something to be said for buying
a 4-5 man "now" and buying
the second one in 18 months (half of whatever the mfr's period is), so they don't both require certification
at the same time, and you at least have one raft on the boat
at all times, rather than out for service for a month.
Easier for a shorthanded or injured crew to launch a smaller lighter raft, too.