As a SPG Plat, I must say there are great options in Taipei, and all the Starwood Properties there are very good.
While food offerings in Sheraton Taipei's Club Lounge is very good (probably the best among the SPGs), but I had to fight incredibly hard to get my upgrades in Sheraton Taipei, which I hope not to be the case in my next visit to Taipei in future.
If the upgrade is to a Director's Room, this is good enough for me. Diplomatic Suite is of course even better. Let's hope the upgrades will be more consistent and not needing Plats to fight for it.
On the other hand, I had really fantastic Suite upgrades (beyond standard suite once!) at Westin Taipei & Le Meridien Taipei is very nice as well, and I got a Junior Suite upgrade there without fuss. W Taipei is my favorite city-based W in Asia at the moment, and if you score a Suite in W Taipei, it is really outstanding. I have no complaints about Westin and Le Meridien's Lounges as well, as Westin serves a good number of hot dishes and daily soup pot, and I remember Le Meridien also having a daily soup pot, and a good range of drinks, including the very nice Taiwan Beer (Gold medal). (but do go out and eat at night in Taipei - the Night Markets' scene in Taipei is a highlight.)
Sheraton Taipei is the most dated among the Starwood properties. But if one managed to score an upgrade is to a Director's room or Diplomatic Suite, then I would might just be best value among the 4 properties. (Westin could be extremely good value too if you managed to get a good rate.)