If sitting outdoors is really important to you, I would opt for the Plaza Inn. Being a CS location, you can usually select your own table. Carnation is a TS location, so you have to request an outdoor table and they aren't always able/willing to take requests (I've only been there once when this happened, but the CM refused to take a seating request; most of the time, they'll just tell you it'll mean a longer wait). So at Carnation, there is a chance you could end up sitting inside. In terms of the view, both the Plaza Inn and Carnation have some tables with an excellent view of Main Street and the park, but other tables have an obstructed/obscured view. At Plaza Inn, I would avoid sitting where the shrubbery would block your view. At Carnation, you really don't get to pick the table, so you could end up in the front or the very back of the outdoor patio.
In terms of food, both are great! I really like the fried chicken dinner at the Plaza Inn; I'd say it's one of the most popular dishes (here on the Disboards). The pot roast is also pretty good.