Depends on what you want to do. Are you running the appliance or your own hardware? That will determine if you can 4.5 or must choose 4.1.8.
Do you want to be able to do more with the macs. 4.1.8 allows the mac client to be distributed but 4.5 actually lets you run checks and remdiation for antivirus/antispware. I am dong this now but have the check for a/v running in audit only mode. Also are doing any out of band deployemt. If you are 4.5 adds alot of improvements for oob. Lastly do you need any kind of win 9X support. If you do 4.5 is a bad choice as it removes support for 98,98se, ME and NT 4.
