As someone who has stayed at both props, I want to remind the OP and others that you are choosing between two excellent hotels in the SPG system. You can hardly go wrong.
There are pros and cons to both, bottom line is that you will be in a great hotel, amazing city and treated well by Starwood at either.
That said, I agree Westin is a bit more "Americanized" but they have been VERY good to this PLAT.
SRG is one of the nicer hotel I've ever stayed, and the location to Termini and equal distance to many sites as Westin is another plus.
I might add that the Westin seems a little more family and the SRG is more business-world-diplomat-y.
Enjoy Roma!