I assume from your location that you're planning to sit the exams in the UK. Given that, I'd go straight for the Bristol DB. Note that there really isn't much need to start hammering questions until just before you write the exams (ie. you've finished studying the material and need to brush up for the exams).
I personally found aviationexam pretty poor, but this was over 18 months ago, so they may well have improved. The cost of a 3-month sub to Bristol is less than it'd cost to resit an exam - that makes it worthwhile in my book!