1) suite upgrades - Sheraton (No contest). They go out of their way to upgrade Platinum's usually to jr. suites (at a minimum) but give out suites and at a minimum club floor. Usually the front desk people are very nice and sometimes even provide options or you can politely ask and they will search the inventory for you. The Westin has very few suites (and they aren't that nice anyway). They book a lot of conventions and professional teams that usually take up this inventory anyway. If you do get the rare upgrade (I got it 3 or 4 times in 3 years!) consider yourself lucky.
2) wireless internet speeds - about the same I think. Nothing major in difference that I recall. At the Westin choose the 'pay' version for a Platinum and they won't charge you. Same goes for the bottles of water in non-SPG rooms. No charge for at least Platinum's.
3) renovated rooms (esp LCD/plasma TVs) - both have renovated rooms. The Westin reno is a bit newer but the TV's have limited channels and not sure they are in HD anyway. One of the most annoying things is that it takes a very long time to change channels so much so that surfing becomes painful.
4) shuttle service - both provide but the drivers at the Sheraton are much nicer and willing to drive you to further distances. Note that in AM rush hour the Sheraton runs every 30 min. into downtown. On Demand for the rest of the day. Westin is always on-demand.
5) in room dining menu variety (am a veg and looking at low fat options) - It appears that the Westin has their same menu as their other properties. Honestly I don't do room service so I can't comment too much.
6) gym facilities - Westin by a mile. It is a mini-health club. Much nicer than the average hotel gym. The Sheraton gym is more traditional and limited compared to the Westin but most people will be able to make-do.
Also note that The Westin is walking distance to the major consulting projects in town. The Sheraton will require a shuttle (it is walkable but a bit far when cold). The shuttle can become annoying when you can walk to the Westin in 5 minutes.
Most likely you will experience both hotels since you plan to be there for a year. Try them both out and see what you prefer.