One thing to know about REI is that they will take back almost anything if you don't like it-no questions asked. If you do take the time to go to REI, they should have demo models so you can see how they work. The REI's I have been in all have a test bed set up for the stoves they sell.

As far as the canister, I haven't done any tests, but I don't think the orientation of the canister makes much difference except in winter. Different manufacturers sell slightly different mixes of butane and propane which have different boiling points. Propane works better in winter, because of its lower boiling point, but has to be mixed with butane in the low pressure camp stove canisters sold for backpacking stoves, unlike the big steel tanks you see for the big Coleman stoves or gas-fired barbeques.

If you boil off all the propane in a canister in winter, you may wind up with some butane left over, as I understand it, but again, this is a temperature issue that you may not encounter.