Hey, miss alex.
I've never used the Tamron 90/2.8 Di, but I've heard only good things about it. Very good things. I think as far as pure value is concerned, the Tamron 90 might be the way to go.
The Pentax macros are supposed to be great, too.
I just picked up a Kiron 105/2.8 macro, and I love it. It's manual focus, but for me, for macro work, that's a plus rather than a minus.
On the other hand, if you want to use the lens for portraiture or other purposes, you might miss the AF on an older lens such as Kiron. So you're probably right to think about the Tamron 90/2.8.
Besides, the Tamron 90/2.8 is way easier to find than a Kiron.
Let us know what you decide on. . . .