I'm researching this just now -- will write something up soon. So far:

PayPal can be a pain, and I've had issues with the support for PayFlowPro back when it was Verisign.

Authorize.net is the heavy -- they are behind a lot (most?) of the payment processors out there, like Chase/Paymentech. Their API is nasty and their approval process can be cumbersome.

TrustCommerce has a very nice and documented API. For basic subscription charging I think they might be the winner. I am still evaluating their support, sales, fraud protection, etc.

Google checkout is nice but no support for automatic subscriptions.

Amazon FPS is very very nice -- especially if you are accepting payments from overseas and from consumers -- but a lot of people don't like whisking the user off to another site.

If possible I will do an A/B test of TC and Amazon to see if there really is difference in conversion rates, errors, fraud, etc.

There are two categories: companies that put their own brand on the experience, and companies that don't. There are advantages to each, and PayPal makes it confusing because they have a product in both categories.

Assuming your asking about traditional merchant processing (the latter) I'd recommend CyberSource, combined with your own merchant bank that actually knows you (not one of those "specialized" merchant banks that just handle your credit card processing.

Braintree is awesome with Rails apps (and particularly rock solid if you needed to implement recurring charges). The only issue is you would need to process $1+ mm through your site, otherwise, there'd be an additional monthly charge. Highly recommended however.

Also, I'd assume that you're integrating the APIs through ActiveMerchant? I'd definitely use that regardless of which reseller/merchant provider you choose.

Quick question: how much is the additional monthly charge? I've got an application on the back burner I was considering using them for, largely on the recommendation of the guy who wrote the Rails kit for credit card processing.

When we had started back in Oct - the quote was an $200 additional fee on top of the percentages. It's eliminated as soon as $1mm is processed

That's what I plan to do, yes.

We use MTrex. It was created by some people who worked on Autorize.Net. I've met with the CTO and he seems to know what hes doing. It has most of the same features, and I have enjoyed their tech support.

They have a nice api and their website is easy to use also. I have used authorize.net in the passed and this has been a little bit easier to use.

Does anyone have any recommendations for credit card preocessing in UK? It looks like I will go with PayPal to begin with, but I am also very interested in looking at other alternatives.

I currently use Authorize.Net and it's not bad.