compare the specification and suggest which one is better for F5

on your existing platform, you can improve performance only in regards compression and SSL. This by adding the corresponding licenses.

If you want to buy a new platform, you should check the datasheets. New platform generation have big performance increase in SSL (already with base SSL license). I think a 5050s is somehow comparable in L4/L7 throughput with a 8900 but with much higher SSL perf. With Max compression, it's a little lower than the 8900.