I have been working with TrendMicro with great success for about 6 years now. Before that worked mainly th Symantec.
At that time, the main reason for the change was the heaviness of the Symantec products.
I think now the things are the other way around, Symantec being a more light weight product.
Recently I was told here in ITKE that Nod32 is performing very well all the way particularly with Exchange.
I suggest you to download the free trials every vendor have available and do yourself some tests.