I did a similar trip a few weeks back with work in Buffalo and hotel in Niagara Falls, Canada (except stayed in the Sheraton).

No problems (with time or customs) crossing the Rainbow Bridge in the morning or evenings and I assume it will get even easier at it gets colder.

In your pricing model, dont forget there is a bridge toll each way (I think $3.25 US) and I'm not sure about the Marriott, but the Sheraton charged $15/day for self-parking.