id probably say go at least 55, but if you can afford everything there is to it, go as big as you can, youll be happier down the line, i am on my 2 upgrade, from a 10 gallon, to a 20 gallon, to a 40 gallon, and the old 10 is now the refugium for the 40! id definetly say go with the "natural filtration" method if your gonna do a reef, its much easier to maintain. if you go predator, there are a few thoughts as to what to do, i personally treat my tank like its a fw tank, no inverts (i think there is a snail and a hermit crab in there for now) lots of flow, coarse gravel, with little depth, that way i can g/w my fish are messy! not the most effective way, and definelty more time consuming, and possibly dangerous (i have a lion, and dont like puting my hands in the tank) but it works, and its cost efficent! oh some very pourus live rock, and purigen also go a long way.