as for rarer, well you're not going to earn extra "exotic" points for a rare rock. generally, people will choose fiji rock, simply b/c it's porous and light weight. hence, more room for bacteria to grow and also more rock (size wise) for your money. what people will often do, however, is to buy pieces of those other rocks that they think look nice, as spotted in the lfs, and work them into their aquascaping. for example, i have mostly fiji, and i have worked in a few branching tonga pieces. it's really up to you.