You are right that only the amount above the tax bracket jump is taxed at the higher rate. With an RRSP, you get tax savings now, but you pay the tax when you take out the money. If you put it in from the lowest bracket, you never save that much on taxes because when you take it out it'll be at least in that bracket, if not higher. So for low incomes, TFSA is better. You put in post tax money, but you never pay tax on it after that. Since you're putting it in from the lowest bracket, that's the minimum tax you can pay on it ever. That's my understanding at least, if someone knows better please correct me.
I think the optimal strategy is that while in the lowest bracket, max out your TFSA first, then contribute to your RRSP if you can. Although if you have pension match, get that first even though it comes out of your RRSP room, it's free money. Above that, contribute enough to get down to the lower bracket. If you have enough to max your TFSA and your RRSP, you're doing great. The important variables are your current income and tax bracket, and your expected income and tax bracket when you'll be withdrawing. If you expect to have higher income in retirement, I think you'll end up paying extra in taxes compared to the tax savings when you were stashing.