Neither! Sell both and get a 50i and tow with that
Only kidding…sounds like you already have 2 nice X5's. The diesel would probably be the better choice but your 35i is newer and sounds like you'll keep it longer. Given that, I'd put the hitch on that one and just go with it. Might not be the best towing vehicle, but it'll get the job done and you won't have to worry about moving the hitch from one vehicle to another.