Having had 15" wheels since owning the F and having just changed to 16" wheels, I cannot believe G0RSQ's comment above that he couldn't feel any difference.
Maybe because I have just changed wheels and done a journey I do regularly, knowing every inch of the road, I can say without doubt that I felt a big difference.
First thing you notice is that the ride becomes firmer, you feel more of the surface of the road and the noise going over bumps or potholes is louder. This I was prepared for and expecting, however, from a comfort point of view I much prefer the ride. There is less 'bounce', the ride is much sportier without being harsh. The front tyres are now wider, 215 as opposed to 195 on the 15", and have made the steering feel slightly heavier. I feel I have more grip up front.
At speed on the motorway the firmer ride is definitely beneficial. There is a stretch of the M62 which undulates and previously the car would bob up and down, making you feel like you were at sea. With the 16" wheels the car just follows the undulations without the extra bobbing.
With 15" wheels the F looks original and if a more relaxed style of driving is your norm then I would say stick with the 15's.
With 16" wheels the car looks more modern and if you like to drive it like a sports car then the 16" wheels would be preferable. Choosing the 11 spoke design also allows you to fit the larger AP Racing calipers up front without the need for any spacers.
scubaman, you should ask yourself what your priorities are and make a choice based on them. Which ever way you go get the top down and enjoy your MG.