Identifying the right F5 model for ASP.NET website

There will be many factors you need to consider. You could use a tool such as Piwik or Google analytics, or even Cacti to try and gauge the needed performance.

You should measure statistics in your site as hits per hour, number or percentage connections using SSL. It you make use of slow changing content then you may benefit from caching/compression.

What is your expected growth over say the next few months or year? Do you have any specific needs that may warrant the use of modules like AAM, ASM? Do you have security requirements? Global load balancing?

You could see if f5s performance tools can help: http://www.f5.com/featured/performance-tools/

Lastly, what sort of budget do you have? That will usually dictate the platform you select, I've found.

Hope this helps.