Well I haven't had much experience using shopping carts myself (hopefully somebody else with some experience will speak up as well) but as far as a user goes, I'll give you my thoughts.
If you have loads of purchases and traffic coming through, then I would say to invest the time and money into getting a shopping cart. The additional costs that may come into play with getting a shopping cart setup would be getting a dedicated IP address and an SSL certificate. You can get some more details about that here:
http://desk.lunarpages.com/faq.php?do=article&articleid=31Now if your buisness is a little less popular than some of the big names, then PayPal might work best for you. Just from personal experiance, seeing the PayPal option there makes me feel a little 'bit better than just going through Random Shopping Site number five. The reason is, PayPal is a name I know and I trust.
Now as far as security goes, I haven't heard of a lot of folks having any problems with them. Fraud is going to be a problem with any Web site that takes in money, you just have to make yourself more aware of it and double check anything that looks or sounds fishy.
With most shopping carts - unless you do some heavy duty editing you are going to be kind of forced to use their basic layout, and then you can do some customizations on the side. Now somebody who is gifted in the template editing and customization might say it is a piece of cake. For us regular folks though, it might be more work than it is worth.
I would say if PayPal is doing the job for you right now, there is no reason to break what works.
I hope that helps and if you have any other questions we would be more than happy to help you out and answer them for you.
