It's like comparing Yahoo to Google, where Skrill is Yahoo, Google is PayPal. Both are search engines, but no-one really uses Yahoo to search. I think Skrill (formerly Moneybookers) is somewhat more widely used in the UK and in India. And i also think that it's fairly popular among internet marketers, but if you take a look at the general population, they'll have only PayPal mostly.
Unless your target audience is a country, which has its own, dominant payment processor(s) (like Russia), as a merchant you cannot afford the luxury to not accept PayPal payments. If you want to avoid using a PayPal account when accepting payments, you should look into payment processors, which can accept PayPal payments e.g. 2Checkout, otherwise you'll leave a good chunk of money on the table.