By the sounds of it you will be better off getting a Thorpe Park pass, espcially with the offer at the moment which will get you a Thorpe pass for just £53!
One thing to bare in mind is Fright Nights but then at just £5 usually, I cant see you using it that much to make it work out more than a merlin pass
http://thorpepark.com/prices/annual-passes.aspx?tabnum=2