Paypal can be reversed quite easily, and if it's payment for services the vendor cannot defend it. Oh and of course Paypal have been known to close down accounts used for 'adult services'.
Regards credit cards, I have it on reasonable authority that the transaction charges are higher for 'adult service' providers - presuming you tell them that's what it's for. The figure is approaching 10% supposedly. The same source also told me that the level of charge backs was also pretty high, to the point where they generally tried to avoid taking payment by card. This was however customer not present transactions(an agency taking payment over the phone), an indie would have more solid footing since she'd have a CHIP and PIN machine. Only problems she'd then have would be setting up a business account, and of course the outlay for the machine. Since most clients wish to pay in cash, then probably not worth the hassle.