more flexible - yes
more reliable - depends on the user and hardware
more secure - no. Even windows can be made very secure by an experienced admin, if you don't know what you are doing anything can be compromised. This report may surprise you
http://www.theregister.co.uk/2007/10/03/ebay_paypal_online_banking/So linux is responsible for most botnets, not that secure then is it?