If you're looking for "different," Hart would probably be your best bet, as its flavor is very much unlike any of the others.
Kary, in my opinion, is the best of them all. it boasts high brix (for a carambola) and ultra high production on a smallish tree. My Kary is currently about 10 years old and still a very nice compact tree with near zero pruning.
Bell has a slight edge in terms of flavor over the kary, but it's less productive (in my yard) and a more vigorous grower.
In general, I don't recommend more than one carambola tree. They produce insane quantities of fruit multiple times per year. And, given that carambola flavor is reasonably underwhelming, it's quite easy to become bored with them.