I got it, more for the convenience of just paying a one-off fee. Be careful though, as there are two different 12-month packages. Apparently, there is a standard one, and then there is the premier club one. I did not realize this and just saw "12 month membership" and went ahead and paid with my Paypal. 3 days later and my Samurai extras still hadn't come through. I sent an email to Jagex about it and they informed me I had signed up for the standard one. Kudos to the company, they instantly refunded that payment and I was able to buy the correct one. The response was super-quick, as I had it within 24 hours and my money was back in my account the next day. For me, the discount and the one-off payment is beneficial over paying each month.
As for paying with Paypal, I didn't see it as any extra hassle. In fact, I prefer to use Paypal over entering my credit card details direct, as it seems a little more secure in my mind.
Anyway, hope everyone has a Merry Christmas and a Happy New Year. May Santa bring you everything you wish for!!