I'm Taiwanese and have tried both places, with my mom who is quite the critic (she used to have a Chinese restaurant) so take this with a grain of salt! I hope you know that both places only offer a few "Taiwanese" things and it's mostly Chinese food in different styles (e.g. Szechuan). They do both carry the Beef Noodle Soup but I haven't tried it - usually my mom just makes it for me! Simple concept but as with udon or pho, execution in broth, noodles, and meat can be tricky! If you try it, report back!
Anyway, my mom was impressed with the scallion pancake at Taipei 101 (on the dim sum menu) because it was nicely crisp and not very oily, which she said was tricky to do. The other things we ordered were your standard Chinese restaurant fare. She, of course, thinks she can cook better. It wasn't so great that we have gone back regularly as a cheap Chinese fix (plus it's not that close to me). It seems like the scallion pancake (plain), hot/cold/sweet/unsweet soy bean milk and the noodle soups beef and pork with pickled veggies) are the most "Taiwanese" things on the menu.
Looks like Taipei Cafe has a few more Taiwanese-style items on their Authentic Chinese menu with the different soups (noodle and otherwise), mapo tofu, taiwanese-style rice noodles. Pretty good portion sizes, not too oily. Same thing as Taipei 101 though. Good but not great. Again, didn't try the beef noodle soup.
As for Grand Asia Market, there is a restaurant inside (to the left when you enter) that is open 7 days a week for lunch and dinner. They serve dim sum on weekends too. I think they probably use the most oil of the three. And the same brown sauce for most dishes. The attached bakery is pretty awesome. The same owner opened a grocery in Durham as well, they also have a cafeteria. It's called Li Ming's. Here's a review: Li Ming’s Global Market | Carpe Durham