I've done both. we prefer treats and seats. At Treats and Seats we were not right up against the rails, but the next row back and it was perfect because we were just under the over hang. I love that your table is reserved and you can leave it and come back all night - and it will still be there. I also like being able to get desserts / drinks right up until the fireworks begin - vs. the Plaza viewing where once you leave the dessert party to go the lawn that's pretty much it- unless you want to lose your "spot". Also, it started raining about 2/3 of the way through the fireworks so since were were under the overhang we were able to stay at our table nice and dry and watch the whole show.
The Plaza has a direct (straight on) view of the castle, but as more and more people finished their desserts, that area became fairly crowded.