They all have their weaknesses and "the best solution"
really depends on your environment. I'm a big fan of both
Trend and F-Secure.

In another post here I outline some of the struggles we've
had with F-Secure and why we eventually turned to Trend
Micro for our environment. Granted, my environment is
nowhere near as large as yours (50 PC's and 8 servers),
but Trend has been spectacular for us.

I'd love to hear more about your experiences with F-
Secure. I believe it's Internet Suite is a top-tier product
and deserving of more attention on this side of the pond.