Like all the other sites, you can join on a system which allows you so many searches for a set fee or you can join for a year, which gives unlimited searching. I've just renewed my Ancestry membership for a year at just under £80. I don't know what Findmypast.com costs for a year though. Since two of us are researching the cost is about 75p per week per person which is good value. Even if you were doing it just for you it would only amount to about £1.50 per week. As I said, that gives you unlimited searching...24hrs a day if you want to.
As to which site is the best, it might be better to buy some credits on one site, have a poke round and then try another. That way you'll get an idea which one suits you. You can try Ancestry.co.uk or Findmypast.com. Both are probably near enough to each other in terms of performance for it not to make too much difference. Marnie might disagree though. Whatever you do, we'll help you if you're stuck.