It all depends on where you are located. In the United States, merchants have thousands of options available to them. Usually if you are processing more than $1,000 a month, a merchant account is usually more feasible.
I would use paypal no matter what the sales were like. It may cost you a bit in processing costs if you are turning more than $1000 monthly, but there are quite a few people who would much rather pay with paypal (and some who will not do it any other way).
You alos might consider 2CO until your volume has been built up. Plus starting & using 2CO might help you review fraudulent transactions prior to using your own merchant account. This will help cut back on your chargebacks with your merchant account.
3rd party payment processors are mostly unknown to potential customers, but I agree you should offer a credit card payment option that does not require signing up for an account.
Just a note - I think it was already mentioned, but in case it wasn't clear - PayPal does not require buyers to sign up for an account. The PayPal Account Optional checkout is available for Buy Now and Shopping Cart payments (but not Subscriptions).
Do I understand it right that the PayPal Account Optional checkout is only available with credit card payments? Does PayPal accept credit card payments using the PayPal Account Optional checkout feature for all countries or only for countries approved by PayPal?