the best way to decide is to go to a store that has a large magazine section (and - of course - several different quilting magazines). don't be shy. browse through each one. write down the names of the mags that are most to your liking.
they're all quite expensive so also visit their websites to browse through their free online offerings. the most useful magazines are those that have both great printed issues and lots of free extra goodies online.
when you have narrowed it down to the one (or more) you like best, take out a subscription. you will save about half the cost of buying each issue from the store.