There is no question that the Springfield gateway is the political no brainer. With Amtrak, Mass DOT, all the local regional planning agencies, the City of Springfield, and MGM too all in bed together for a Springfield to Boston rail extravaganza, I can't say there is much political capital in even thinking about the Greenfield Gateway.
However, from time, cost, and logistics points of view, the Greenfield Gateway offers a much quicker option for those residents in Western Mass, Southern Vermont, and Southern New Hampshire who seek more direct rail connections to Cambridge as well as the North Station/Downeaster.
The cost to make it happen track and signal wise is certainly in the same ball park from Greenfield to Wachusett as it is for Worcester to Springfield.
The achilles heels for the Springfield routing include heavily constipated CSX operations, the Framingham grade crossings conundrum, and severe capacity issues at South Station. While all of these "issues" can be dealt with by pulling out a very big check book, in my opinion the time, money and politics to pull off the Springfield vision is many many years away from being a slam dunk rail reality.