The twin air is great in your 50mph scenario. There's plenty of grunt, 4th or 5th would be fine.
Take both motors out for a drive and see. The twin air is much more fun and a fair bit quicker.
Mpg wise, the multijet would be my choice. fun and performance = TA.
I've can get 70 mph in 4th gear so it has got grunt and if you drop the speed below 60 mph you can get Good MPG out of the TA engine
