I agree and disagree with scottychaos on UGF. When one's budget is tight or one has a small tank, an UGF is the best way to get a good biological filter. Yes, they collect the "mulm" that forms in the tank but "storing" that mulm in the gravel as they do is not much different than storing it in an outside power filter or even in a canister filter. The tank water is still in contact with the "mulm". Now this mulm is the solid waste and other stuff being broken down into essentially dirt. It is not inherently a bad thing but becomes one in a small body of water (a fish tank). Vacuuming the gravel when one does a water change takes care of most of the mulm -- about as well as the usual outside power filter. If one does weekly water changes of 50-75% then very little mulm is left in the gravel. When one is doing regular partial water changes there is no need for carbon in the filter either. The water change does what the carbon does.
I have never been a big fan of power heads with UGFs as they suck up small grade gravel and ruin their impellers and their force does create packing in the gravel bed and thus poor flow and filtering. Air driven UGF's rarely pack and have nothing for gravel to ruin. The air lift type work better than powerheads for biological filtration as biological filtration does not need force and speed -- just steady free flow -- to work. Power heads are overkill in a small tank and in a large tank a canister filter is better.
UGF's for small tanks have lost popularity in fish stores as they don't need expensive cartidges every few weeks so pushing them costs the store profits. Fish stores make alot of money off the pre-made filter pads almost all of today's outside power filters require. These filters were once simply pumps and open plastic boxes that could hold whatever filter medium one wanted to use -- like cheap bulk floss and carbon or even peat moss. Today's outside power filters can only hold the company's ready-made cartridge insert and trap you into spending alot of money every year buying them. Stores love power filters for this reason -- $$$$$.
Sponge filters also, unlike UGF's, wear out and must be replaced. UGF's work without additions year after year. One does not need the little carbon inserts for UGF's if one is doing water changes, so there is nothing to continuely buy for an UGF except repalcement airstones once a year -- $2.
I won't use an UGF in a saltwater tank.
The best cheap non-UGF biological filter is the Jumbo Lustar Box or Corner Filter. Few stores carry these but they are available on line. These BIG inside corner filters are versatile and well made. I put an inch of floss in the bottom and then fill them up with small grade aquarium gravel or Eheim Ehfisubstrat for a biological medium. Every couple of months I change the floss and wash the medium in de-chlorinated water. One of these filters can do a 29 gallon -- 2 if it's heavily stocked. I will use 3-4 in a 55 gallon. I can have extras in other tanks cycling so I can instantly start a tank. The filter lasts for years as does the main medium. It costs about 25 cents to replace the floss. These filters date back to the 1950's and are very popular with breeders.
Eheims are the best outside canister filters. They are more expensive than others but they last forever, do a great job and cost pennies to clean and change when used as a biological filter -- again, even here, all you change is a bit of cheap bulk floss. You don't have to use the company's pre-formed pads. An Eheim -- after a year of use -- will be cheaper than the cheapest outside power filter because you are not forced to buy all those pre-formed filter cartridges. Big savings and better filter!
My favorite outside power filters are the Emperors as they have the best biological filtration feature for my money.
My 20 tanks are bare bottom too -- the outside bottom is painted black and I have clay pots full of live plants in all tanks. The filters are either the Jumbo Lustars or Eheims -- depending on tank size and fish density. Trapsrock needed some good basic equipment that would do the job and I think people he talked to on the fish sites focused on his problems and ideal solutions more than giving him real solutions that he could do ok with right now. UGFs, in the long run, are bad for a Devil because Devils dig, uncover the filter plate and thus ruin how the UGF works. Devils have to have canister or Emperor filters when they are large and dig.