The higher up they are mounted the better.
I have read loads of reviews of radar reflectors and some of them are next to useless.
Probably the best overall is a large octahedral type - like you see on buoys etc - they aren't easy to mount on a RIB but they are dirt cheap.
If money is no object the See Mee is the best - it's an active design that make your RIB look like a much bigger boat.
The Firdell reflectors are good but quite bulky.
I think the best type for a RIB is probably the trilens - easy to mount and looks cool - most tests reckon it's pretty good as well. Costs about £120