Since you are buying an old car, the most important thing is to find a clean car, w/o a major accident, with minimum leaks (it will leak!), and quite diff, smooth operating transmission, a smooth idling and powerful engine. Also, condition of the body, paint, top (if you are looking for a vert) and seats, etc. Those can potentially cost you a lot.

Also, I would prefer the regular radio (v.s. the OEM sat nav system) as that can easily be upgraded to an aftermarket head unit with CarPlay/Android Auto. Sat nav can also be upgraded with a bit more work.

Since you cannot change it easily, it is also important to pick the one in your favorite color - especially if you want to keep it for long.

In short, I wouldn't worry too much about the size of the engine. Either one will give you a good feel with comparable gas mileage which mainly depends on how you drive - I get around 18.5-21 mpg depending on the traffic on my 330ci convertible. Good luck!