anyway, if I were to choose between the DA*50-135 at 135mm and F/FA135, I will choose the latter. the F/FA135 are AF speed demon on the K-7, much so I assume on the K-5. aside from that, this is what you can expect from the lens.
although, I'm pretty much hard-pressed choosing between the FA100/2.8 macro and FA135/2.8. both are outstanding in their own personal way. although as someone already mentioned, the combination of focal length and aperture speed + focusing distance makes the F/FA135 lens gives it a lil more DOF punch. 100 macro will always be a macro. great CA control and abit sharper and contrasty. bokehs from both are of their own and both are very nice. colours from both are great, punchy and vivid (although I think that the 135mm is a bit more saturated). the 135 is lighter and is IF which is great. the 100 macro is heavy and solid and extends when focusing. AF speed, 100 macro is good, FA135 is very fast. again, between the two, I can't choose which is better. they have their own focal length strengths and uses. colors from both lenses are outstanding.
in comparison with the M135, the F/FA has a half-stop advantage which gives you more light and a bit more DOF. but what is really good about the F/FA 135mm is it's MFD can go as low as 70mm. that is half the focusing distance of the M135 at 150cm and 30mm below compared to the DA*50-135.
with Pentax's shortage or lack of an AF 135mm lens in it's existing line-up, I would say it a a "buy it if you can" lens. the Sigma 150 would be the closest but is not available for the K-mount. besides, the OS one might not be available for Pentax or could cost more.