The Westin is in a nice area, a lil more residential than the Delta and Marriott. You can walk from the Westin to the Marriott via the waterfront boardwalk. It’s nice walk.
It all depends on what you want to visit and how much walking you want to do, Halifax is very hilly and steep, be prepared to walk. I know Halifax very well and have a good knowledge of all Bonvoy hotels in Halifax. Just ask away.

I recently stayed at the new Delta Halifax Downtown and was slightly disappointed; staff was nice, the renovation was nice, but they haven’t finished everything yet, the restaurant and pantry was meh. I don’t think the Westin or the Marriott should be worried at all at the moment.
I don’t plan or returning, I’m just too happy with my usuals spots. Nothing here stands out. The restaurants at both the Westin and the Marriott are in a different league compared to the Delta as well.