Myself have used phpcoin in the past it better that a lot of paid software

But WHMCS gets my vote

I really dont like to much this software, but i think is the better of the free ones.

I used phpCoin breifly once. It was ok. Had a hard time with the backend. For some reason I could not relate to it.

Security should not be a problem. All of the open source communities are generally very quick to spot and address vunerabilities. It would be hard to find a script that has never had a problem.

Personally I use CE. I have two different ones for different bliing accounts. You should pay 5.00+ a month through any of the larger reseller providers. So cost should not be the issue.

The concern you should have is the direction the developer is going. Last time I was at their site, a while back, they were bringing out a more advanced and paid version. There seen to be a lot of concern about how long the free version would be supported. So if you use it, make sure you understand where you stand with that and be prepared to switch later down the road if need be.