We've had lunch at both, and they are both really good. LTT has some great sandwiches and salads (The Tricorn sandwich is terrific), and their desserts are wonderful! The Plaza, though, has all those yummy ice dream desserts, and their Reuben is to die for. I guess it depends on where you will be in the parks at lunch time- really, they are both winners in my book.
One tiny thing to consider- Plaza does have some Main Street characters come in and entertain occasionally. The Mayor is a hoot, as is the lady running for Mayor and her friend, the newspaper lady. Sometimes the Barbershop Quartet even comes in to serenade guests. LTT is nice, but there isn't any chance of entertainment, unless you want to schedule near parade time, then duck right outside and catch it as it goes by- the LTT is a good vantage point for the parade.