Hello theoneyouknowleast and welcome!

As has been mentioned, we have a dedicated forum for those with SPG points (yours is a common predicament for Starwood patrons visiting Tokyo)

Not so long ago the Sharaton Miyako was a category 3 hotel which made cash & points a bargain (2,800points & $45 - the location is also close to my father-in-law, so perfect for me). The Westin was never such great value for me, although I have visited it, so know the location.

The location of the Westin is a bit of a hike from Ebisu station (JR Yamanote) but there are moving walkways along much of the route, so it won't seem particularly far.

The Sheraton Miyako has a regular courtesy bus to Meguro station (JR Yamanote) although you can walk there in just over 20 minutes. As well as this there's a subway line very near the hotel which gives you more travel options.

Check out google maps and see for yourself.

There's actually little between them. Neither are in hugely ideal locations but neither present much of a problem (although I like the fact that Azabu Juban is within walking distance of the Sheraton Miyako as this is one of my preferred areas to hang out in late at night).
I'd suggest you choose based on quality, comfort, or simply what your points will allow.

MUCH more information in the SPG forum:
http://www.flyertalk.com/forum/starw...etter-etc.html
(quite old this thread, concentrate on the later pages)

http://www.flyertalk.com/forum/starw...er-thread.html

http://www.flyertalk.com/forum/starw...er-thread.html