The 2 types of LR you mention are very different. As Thomas says, many like diversity, however, if you are actually trying to achieve a system that is "natural" looking and consistent with the real reef "look", you will want to choose one or the other. Caribbean rock tends to be very dense. It is not as porous, and it is much heavier. Fiji, on the other hand, is very porous, lighter and thus, more bang for your buck. 20lbs of Fiji will get you more pieces of rock then will Caribbean.
You should research the 2 types here and on the internet and see which appeals to you best.