Opinions will very on this, each has pluses and negatives to them.
UGF (under gravel filters) draw junk downward where it breaks down, nice and neat as long as the powerhead/air stone works, for rooted plants this could be both good and bad as it supplies them with food (+), but makes cleaning up the tank a pain(-) as the roots will work thru the filter.
HOB (hand on back) work just fine for a planted tank and there's no real plus or neg to them that I know of.
I just went from a UGF to a canister and already see its negatives as the junk that would get drawn down and into the gravel just sits on top looking ugly until its cleaned up using a gravel vac, or something decides to eat it. If I have to move this tank I'll probably go back and use both a UGF and the canister for that reason, as for the charcoal thing it seems to be a give and take with the bulk on this board saying take it out as after a bit it doesn't work as well if at all and replacing it just isn't worth the $$
