Honestly edX is my favourite site to do these courses. The fee you pay for the certificates if you go that route are indefinite, so you can do the course as you please. Other sites you would have to pay monthly and depending if you don't complete within the month you'll only pay more. So, price wise really good. The courses I've taken are really well done. Quality instruction and depending on the course good forum interaction too. I've also done free courses and are exactly like the payed ones difference being the certificate. So, if you don't want to pay you still get quality instruction.
