I went to both last year, I was unsure if medicine was the career for me so I went to medlink to find out more about medicine and met many students considering a career in medicine like myself. Also there are lots of lectures on various specialities in medicine and you meet gps, hospital consultants that tell you about their job and what lies ahead.. its useful stuff but it is quite expensive and I dont belive its really worth the £200! you could spend £30 on books and search the net and will have more information than a medlink student. They advertise that you'll get to see a live operation but last year our group was one of the last to go in so we didn't get too see much, you see the operatio via a live video camera linked to the lecture room. If you watch that plastic surgery programme on channel five, it pretty much the same as that and thats live aswell.
Medsim is more practical based hand on stuff, its definately useful, I really recommend medsim because it was truly a great experience and when you do your work experiences you see how everything ties in eg patiant diagnosis.
If you can afford it medism is worth it, some of the lecturers even repeat the same jokes at medlink and medsim. Its a commercial money making business but because they are the only ones who do these courses on such a large scale they are popular. In my opinion they certainly aren't popular because of the vital information and experience they give!!