I've just bought a valise. I keep it in one of my cockpit lockers and bring it out and store it under the helm's seat when sailing. In port it goes back into the locker and I can remove the helm seat to walk through to the bathing platform. It gives me the best of both worlds - I get free deck space by not having a canister mounted on deck all the time and the liferaft is mostly in a dry locker.
A shame, I know, to have to take this into consideration, but consider where the boat is moored. If theft is a problem in the area, a valise which lives behind a lock and key may be a better bet than a canister which reamins topside to tempt the light fingered.