They are both full featured pen testing services. I'd personally go with Back|Track as that runs really well from a LiveCD and it is what I know. However the distribution doesn't matter so much as the tools, both contain similar tools, and if you need others it is always easy to install them.
That being said, if you want something lighter you might want to try and build a system yourself using something like remastersys and Lubuntu to only contain what you need, being that both of the distros you mentioned are huge.
