The 1-Series may be BMW entry level line, but the 135i is anything but entry level in performance. It not only can be trimmed to have all the luxury as the 3-series, but is a much tighter, lighter and faster car. The cockpit is essentially the same size as the 3-series and 6'+ drivers will not have any problem with headroom (as with most cars if your closer to 7'+ you will have to do some shopping around and custom seat and rails would be in order).
I have been in the Exige and at 6' 4" you begin to have issues with visibility, Also there will be next to no head room when you decide to wear a helmet and track your car
You will not be disappointed with the exige, just looking at the car puts a smile on my face whereas the 1 and 3 series are so bland most people wouldn't give them a second look. I suggest the BMW M coupe as an alternative to the Exige, great character, comparable performance, more luxury, more room and great fun on track days!