Verotel
Based in Amsterdam, Verotel content restrictions are less strict than their US counterparts. There's also no expensive IPSP fee for processing VISA subscriptions if you go with their 'Ticketsclub' option. However, their admin system is not as versatile and user friendly as CCBill. Their customer support is limited and tech support is not available on weekends.

CCBill
Based in the US, CCBill content restrictions and review policy are more strict than Verotel (for example, they do not allow parodies). A $750 initial IPSP fee is required for processing VISA subscriptions and $375 renewal fee every year thereafter. However, their admin system, affiliate program and program options are superior to Verotel. Their customer support is 24/7.