The best stuff I've ever used is gel that I ordered from eBay! But this was a refill from an old kit that I've had for years.
I've had trays made already from the dentist's office for my upper bridge. For the lower bridge, the eBay kit came with a "wax" type substance to make your own "tray" and it works just fine. I leave the gel on for 15 minutes, and I'm all set.
If you've never whitened before, you'll need a couple of days of usage to get the full effect.
I personally don't like any of the strips that are offered in the stores. I find that they're hard to keep on, and that I drool off most of the peroxide while trying to keep them in place. I prefer fitted trays that hold the gel on your teeth for as long as it's needed.