The problem with these kind of offers is that it's impossible to do a fair straight comparison between the sites. For example you might join Wink Bingo and get a £50 bonus spinning the wheel, but only £10 at Bingo Giving. However, as mentioned above the likelihood is that you'll get the minimum bonus as all spins guarantee a prize. By using this as a metric you can then make decisions as to which site is best to try without being seduced by large bonus offers.
For example, Jackpotjoy offers new players a 250% deposit bonus. Zingo Bingo offers a 200% bonus plus the chance to win between £10 and £2500 guaranteed. Assuming we only get a tenner that means Zingo is actually offering us a 300% deposit bonus. The question for you as a player is which site is going to give me the most fun for the money. It's also important to consider withdrawal terms and conditions too.