I'm in the same situation! We chose TASIS. I think both are excellent schools, though. I had a phone conversation with the admissions dept from each school and used that info to help me make a decision.
Do you know where you'll be living? Proximity to your home may be a factor to consider.
I chose TASIS based on what my expat friends who previously lived in the UK told me and the information I got from the admissions dept. My 8 yr old daughter had her heart set on TASIS after paging through their glossy brochure. And (although not a deciding factor) I am a big fan of school uniforms. Also the other expat families from our company go to TASIS as well.
Good luck with your decision. It's hard to know what to do if you haven't had a chance to actually visit the schools!
If you end up at TASIS, perhaps our paths will cross in the fall.
Jenny