I can't speak for Alteon's lb modes as I personally have no direct knowledge of them. It sounds to me that "Ratio - Node Address" (LB_METHOD_RATIO) or "Ratio - Member" (LB_METHOD_RATIO_MEMBER) should do the job for you.
There are other modes that could also be appropriate. Observed and Predictive modes could be applicable if response time has a correlation to the server capacities.
If you are having questions with BIG-IP configuration and deployment I would suggest you contact our product technical support where you can get more detailed support on the above topics.
F5 Technical Support
Email: support@f5.com
Phone: 206-272-6888
Web: http://www.f5.com/support/
AskF5: http://tech.f5.com/
-Joe
Nimbostratus