Hi roentarre,
I have owned and used both these lenses, both for a short period as I'm not that interested in macros really - they were used for a short term project each only.
Some of my experiences are here:
https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/289-100mm-macro.html
from where I quote myself:
Quote: SMCP DFA100/2.8 and the Tamron 90/2.8 Macro 1:1 Di (the current model, Di means Digital, meaning extra coating on the back of the rear element ensuring no reflections from the AA filter will bounce around).
The winner is the Tamron, thanks to lower price, better resolution at large aperture openings, better build quality, focus delimiter, recessed front element making it unnecessary to use the bulky lens hood. The DFA100/2.8 has a tad better coating and probably better resale value and a ugly bad hood, a plastic build, no focus delimiter and somewhat less resolution at f 2.8 - f8 (as I recall it). For macro shots at small aperture openenings they produced equally good pictures.
Other opinions in the thread of course... To me the SMCP DFA10072.8 was a little flimsy, or better expressed, it felt flimsy. I'm not a big fan of that QuickShift function of Pentax'. I don't like the feeling of it. Still, if you are going to use the lens as a macro only any of them will serve you just fine, just a bit different.
Good luck,
Jonas