I Use the Repti Glo 5.0 Compact Tropical Terrarium Lamp hoocked up to a clamp lamp that i clip onto the cage at a good distance with a another clamp lamp with a basking bulb.
Works great.
What does your current setup consist of?
To answer your question the 13watt CFL is smaller i use the 26watt four my veiled, my panther uses the 13 watt.
It seems that there has been a change in the manufacturing process with the reptiglo compact bulbs making them unsafe to use. (Info provided to me by other members with UVB meters.)
The 26w bulbs are physically larger and emit a little more heat (but still not very much at all). That's the only real difference. The ratio of UVB is the same, it just depends on your setup for which size will work better.