Random thoughts...
- If the 4 cyl. isn't enough for you, there are plenty of options to swap a 6 cyl. into the 318ti.
- My mechanic suggested replacing the radiator and several other parts of the cooling system at 100K
- Since you said that you won't miss the convertible, my guess is that you'll love having all the space in the back seat and cargo area of the 318ti. If the 318ti you're looking at has a sunroof (most do), you'll probably be satisfied by that.
- The 318ti probably has a quieter cabin since it's not a 'vert.
- If you've done your own maintenance on the Miata, there will probably be a learning curve for working on the 318ti but it's doable. Also, if you end up taking the 318ti to a mechanic for something that you'd have done yourself on the Miata, that will make the 318ti SEEM more expensive to maintain.
- And finally... I briefly test-drove a Miata before I bought my 318ti. I think I made the right choice. Granted, the car I was replacing was an '89 Civic Si and the BMW is very similar to the Honda in features and configuration, so I was a bit biased towards the hatchback. The 318ti seems to be just as much fun as the Miata, but a lot more practical.
I suggest that you try to trade cars on a trial basis, for 3-5 days so you have a chance to make an educated decision.
Good luck!