I am very familiar with the three Marriott CBD hotels. Also have stayed at the Four Points Docklands, which I like, but it will not be as convenient, or have such large rooms as the CBD hotels.
Each hotel has its merits and fans. The Marriott is an older hotel, but rooms are in decent shape. Staff are very friendly. It is the only one with an Executive Lounge. The Lounge serves its purpose with drinks and food, but as the above poster says its average.
The Westin is an elegant hotel, caters well to the business market. Rooms are a decent size, but upgrades are mainly to a higher category, as there are very few suites. Excellent General Manager. The only caution would be that works for the Metro are nearby. There are courtyard facing rooms that are quieter.
I guess the Sheraton is my favourite. Rooms are not as big as the Westin. Has more of a boutique feel.
Not all rooms have any view, and those rooms may have construction commencing next door soon.
Again not many suites, most rooms are a similar size.
I do like the location at the Paris end, close to restaurants and theatres.
Rooms are very quiet generally, as it is on Little Collins Street, a minor traffic street.
I think your idea of trying the Westin then the Sheraton is a good one. Make sure you a member of Marriott Bonvoy!