I reckon that the average spend at out-of-town McDonalds is probably higher than those in town centres; note how almost all of the vouchers that they flood the market with (£1.99 for burger and chips, etc. that you see in the Metro, on the back of bus tickets, handed out on the street etc.) isn't valid in outlets with a drive through. In my experience the clientele in town centre branches seems to be a bit more rough and ready as well.
For breakfast, I prefer McDonalds but lunch/dinner Burger King is definitely a better offer though. I didn't use to go there very often due to the prices, but they always have deals on their app as well as vouchers you can print off the internet that makes them very competitive price-wise with their rivals. I find the quality of Burger King to be very variable though, in terms of standards and ambience of their stores definitely worse than McDonalds, for instance the branches are often dirty with litter strewn across the tables, damaged furniture, staff with limited social skills, slow-moving queues, variable drink offering (some branches have self-service drink machines with numerous flavoured drink options whereas others have the fizzy drink-only which are operated by staff behind the counter), different card acceptance, and opening hours that differ between different sources which differ with reality. Went into one branch of BK near a major Underground station at about 1.45am in the summer - the website says it's open 11am-11pm Mon-Sun, the App says it's open 9am-2am Mon-Sun, and the sign on the door says 9am-Midnight Sun-Thu and 9am-2am Fri-Sat (the latter is correct) - the chairs were on the tables, half the lights were off and the staff were vacuuming with a power cable trailing across the floor, but they were happy to serve me. McDonalds seems much better with opening hours that (generally) reflect reality and a more consistent product whichever branch you visit.