I use the 10.0 because I have a large cage and my Chams wander around constantly so I feel the 10.0 is better than the 5.0 due to the fact that it penetrates more and has a higher UVB concentration... so between screen and plants I can be sure my Chams are getting enough UVB as they explore about the cage all day. However if your cage is smaller or your Cham is the type to park it and bask under the UVB bulb I'd say the 5.0 would be good. I used to use the 5.0 but when I took my Cham to the vet at 9 months old I asked him about the UVB because I was worried since I read that natural sun is WAY stronger than the bulbs so I asked about the 10.0 and he suggested I switch given the cage size, mesh screen and dense planting.
I currently have a 18" 10.0 in a 36x18x18 reptibreeze cage with pretty decent vegitation. In about a month when i get home from college i am building him a 48x36x30 cage (or bigger) with 2 umbrella trees and i want to get a 3 foot uvb bulb. Should i go with the 5.0 or 10.0 under those circumstances?