You've narrowed your search to two good cars!
As for the number of posts about problems...
Of all the people who have taken delivery of their Smart, the small subset who had some early problem seek out a forum like this one. The vast majority who are happily motoring are less likely to bother to read or post.
We've only had our Smart for three weeks, so our experience won't be of much comfort. However, our car runs beautifully! No problems!
This car has a unique transmission that has a bit of a learning curve for best performance and comfort. It took me about a week to adjust. My husband and I argue about who gets to drive the Smart because it is so much fun to drive.
If you are comfortable with a 2 seater, I would go with the Smart. If you think
you would be better with a 4 seater, you can't go wrong with the Fit.
Can't advise about the diesel!
Enjoy whatever you get!