I have always used a U/G filter and I have the most experience with "peace river" gravel (second to last on the right)
http
/www.carib-sea.com/pages/products/fresh/freshsub.html
though folks swear by Red Flint gravel as well. It's slightly smaller and it's what I would have used if it was available to me here. These are both small enough that I used a sheet of fiberglass window screen over the U/G plate to keep the smaller pieces from falling through the cracks.
I just upgraded to a larger tank 2 wks ago and I went with a sand bottom, no U/G. I used pool filter sand, which is a graded (uniform size range) and is what I would consider the large end of "sand". 5$ per 50# bag. It would be too small for a U/g. People use "play sand" from the Home depot etc and do fine, but I prefer the more uniform size and did not want the really really small pieces that dominated the bags I looked at. I looked at a lot of sand while waiting to set up my new tank, and even all pool filter sands are not of the same size or size-consistancy. Keep looking.
My new tank looks awesome, but I can already tell that I am going to miss my U/G. I could go weeks at a time, if necessary, with minimal care needed. It worked flawlessly. The new tank has major flow and a long overflow going down one of two drains each with a filter sock, to a monster sump with bioballs.
Those filter socks are the only mech filters I have. They work great, but require cleaning 2x/wk ( I have to put them through the washing machine to even begin to adequately clean them, so its not an entirely trivial exercise. I do have a rotation going, but still. . . I have traditionally been a overfilter-understock kinda guy, so I sort of resent having one more responsibility.)
There does colelct a layer of snail/fish poop and other debris which is easily siphoned off the top. The sand is remarkably resistant to siphoning-off and little is sucked out if run the vac ovover the surface. There has been no need yet to shove the vac into the sand, but by pinching the tube and slowing the flow, much of the smaller crud can still be siphoned off with little sand loss. Regardless, sand is cheap and I bought two extra bags for just such a need. I expect it will a year or more, perhaps much more, before I'll need to replace some.
It is definately nicer looking than any gravel I've ever seen, though the two I mentioned above really are great choices. I know I'll miss my U/G, but the aesthetics of the sand and the lack of lifttubes and power heads make my sand bottom a keeper.
