What kind of airstone do you use?

The only one I have is for my goldfish tank, a long bubble wand

www.bluewhippet.net

What kinda filter do you have? If its a HOB you shoudent need an air stone.

Whatever shape you choose, you'll get bubbles - the question is do you want them in one concentrated spot, or spread out?
With the longer airstone, if you have a strong enough air pump, you'll get a little wall of bubbles (I'm thinking of the actual stone, usually blue or green, about 4-5" long - a bubble wall will do the same thing but with finer bubbles). If the pump isn't powerful enough, the bubbles will come out of only the first part, giving you pretty much the same effect as a small cylindrical or spherical airstone.
With the small airstone, if the pump is too powerful you'll get a lot of bubbles concentrated in one place, and a great deal of surface agitation. It's not very pretty, and can splash. If you don't want the bubbles spread out as they would be with the long airstone/bubble wall, you can bleed some of the air off with a valve.
If you've got a small pump, I'd go with the small stone. If it's a larger pump, it's just a matter of personal choice.

None at all, nada, zilch. Airstones to me are ornaments which make noise, blow off CO2, and splash water. I neither need, want, nor like them. With modern filtration they are not necessary. If you like the look, fine, personal choice item.

You can use just an air hose buried under a rock or a piece of driftwood. The bubbles can look good on some set-ups. Others have mentioned the need for "skimmers." With bubbles, even a few hours at night on a timer, the surface will be disturbed enough not to need a skimmer.

RTR, I don't like them either. I have them on 2 tanks and not one one. I tried to take them off and it looked like my fish were going to the top for air. Maybe I'm just paranoid. I have HOB filters. Does the water level have to be lowered a bit to make a small splash? I have the water level as right below the bottom part where the water falls out.

I used to use one for my 55 but I turned it off a week ago. To me, I would mostly use them for speeding up the water aging process in a drum and dont mind them in my 55. But since I have wet/dry systems on my tanks it was just a waste of energy.

You should not need airstones at all. I have the water level on my 90 right to the top and the fish never have a problem. I run 2 HOB's and a cannister. My kid's 10 gallon only has an HOB and the fish are fine.

Do yourself a favour and skip the air filter. IMO aquariums are much more relaxing when you can look at them and not hear the constant noise of an airpump.

I tend to only have the one filter system, so I have air stones as a backup circulation system should the filter pump fail. Personally, and it's just my opinion, the air stones are a nice gentle way to set up a circulation current in the tank to stir things around and aid the filter. you can also use a powerhead, but it can mean some areas have really strong current. Since I just trickle the water back out of the wet/dry (above tank) it doesn't circulate the water much, so the airstones do that, and I don't mind the look and the fish seem to like swimming in the bubbles. I don't need the bubbles for O2 as I'd get plenty elsewhere like in the wet/dry, unless the pump fails and then it's a backup. Since we are home most of the time any sort of failures aren't a big problem anyway...