I don't use the UVB/UVA lights with my torts, but seems to me I remember the 10.0 being the one most people use. However, the distance from the substrate to the bulb is what's important. You don't want to have the bulb hanging too close to the substrate.
I currently use a 100 watt mercury vapor bulb (MVB) with my greeks that is placed about 14 inches above the substrate. When I used the Florecent uvb bulbs I used an 8 or 10 and I had it at least 12 inches above the substrate. Here is a good site that gives lots of info on uvb bulbs http://www.uvguide.co.uk/fluorescenttubes.htm
The ReptiSun 10 is a good bulb, but the ReptiGlo 10 is not. It has a history of causeing eye problems & other health problems. The ReptiSun 10 should be 8" from your tortoise and needs to be replaced every 6 months. Also coils & compacts have a bad history, so steer clear of them too.
Good luck
Patsy
I'm a fan of the combined heat and UV bulb such as the Powersun or MegaRay. The 100W MegaRay tells you the minimal basking distance is 12" and this will not produce an excess of UV and the box tells you how much.
Danny who sometimes posts here advised us to run the MegaRay in for 12 hours before using it for our tortoises. I just did that with a new MegaRay and measured the amount of UV coming off it. There were huge fluctuations in the amount of UV coming off the lamp until it settled down. But I'm rambling now..........
Jonathan
I guess it depends what kind of tortoise, but for my red footed tortoise I like using Zilla's desert 50 bulbs. I have been using them since last year. My red foot is in shape so I can't complain.