ski in france

I am wondering if it is cheaper to buy individual tickets and get rentals a few weeks before you go to your resort.

Are you an independent traveller or are you going with a tour op?
In either case it is generally better to reserve lift passes and gear in advance. Your tour op will have a good deal, some even better, like Peak Retreats who are not adding the latest VAT rise to their lift pass prices.
For an independent traveller, go to your resort website and see if they have any offers for booking your pass online. You can also check out the best deal for you whether it's a family pass, the full domaine pass or a more limited area. There are learner passes which may be better value for a few days than a full pass for example.
For gear, check out the hire options online and reserve your gear ready to pick up at the resort. Your tour op will normally offer this service. For couples or groups, check out Ski Republic who have a two for one offer - even if you're a single traveller they will try and pair you up with someone else.

Depends which resort you are going to .... the Immobillarie from which I booked the apartment in Chatel has offered a 10% PDS lift pass discount ... happy days 8)