Welcome to FT, one! I've stayed several times at the Westin and I liked it. It is about a 12 minute walk to Ebisu station using a series of 5 moving walkways that operate from early in the morning until about 11pm. When they're shut down, the walk is about 15 minutes.

Looking at the map, the Sheraton Miyako is about a 5-7 minute walk from two subway stations on the Mita line.

Both hotels are in the same general part of Tokyo but I would say that Ebisu is somewhat better connected to the transit system than Shirokanetakanawa station. Only somewhat, however, so either would work on that regard.

Not having stayed at the Sheraton, I can't compare the two hotels but I strongly suspect the Westin is the better of the two.