See how many hours you're likely to do in a year before you commit to purchasing.
(Very) roughly speaking, if you do fewer than 20, hire. If you do fewer than 200, share. If you do more than 200, buy.
There are exceptions to this rule. If it's a rare aeroplane that you really want, you may have to buy it (you could always syndicate it afterwards). If it's an LAA type, the numbers above probably reduce a bit.