I leased not knowing whether I would buy it or not. So neither poll option applies.
A number of factors went into my decision, ultimately, to buy it off lease (which, incidentally, I just did early and well prior to lease expiration). I wanted to see the F30 and 'listen' to the rumors about the F80/F82, especially the engine, I wanted to drive mine for awhile to make sure it had no bugs, rattles, recurring problems, etc. (it did not), I wanted to spend some time in a 335i to judge the turbo lag (yes I realize the F80/82 may or may not have the same amount of turbo lag given the different configuration), I wanted to think a bit about the NA versus FI option and I wanted to make sure I was into the long term relationship with my car. And with a second DD (a Jeep Wrangler) I was able to keep the miles on my M3 very low but still enjoy it.
And I was willing to pay the lease versus buy premium to have these things.
Yes I realize much of this sounds like dating...
But now I own mine and could not be happier.