Well, I have decided not to go for a sit down meal, because we have over 120 people attending and our service is not until 3pm - which means based on my time plan we wont get to the reception until 5:20 - once dinner has been service etc a good chunk of my wedding day will be gone, so instead we have elected for a buffet meal, to be served at 7pm. (Much to the disgust of my mother!)
If you really wanted a sit down meal you could speak to caterers once you have your selected venue and ask for 2 different choices - it could end up a logistical nightmare though, and your venue may be unwilling to let outside caters in.
Saying that I work in a manor house that is rented out at weekends for weddings and whilst they usually insist on having internal caters only if there are special dietary requirements which the chef cannot meet (eg we recently had an indian wedding) then they have let up on it - this could make it a lot cheaper.