I was wondering this very same question for a while myself. I would like to add my experiences to the post.
The Pentax Forum (showroom in Tokyo) does a lens rental event. Basically you can rent a couple of lenses for free for two hours. I rented the 70 and the 77 to see which I liked more for the kind of photography I like. I looked at the pictures and I think I've settled on the 70 although it is a hard choice. The price is great and I really liked how the pictures turned out. Also since it was a little wider this is a plus for me. I like to shoot pictures in cramped little narrow alleys.
Bokeh Comparison - Unfortunately it didn't dawn on me to take a similar picture to number 02 with the 70mm until after I got home. Boy was I kicking myself. But shot number 01 has bokeh in the background. In this comparison I think the 77 is pretty nice. I would liked to have seen the same scene done by the 70mm.
70mm
77mm
Portrait Comparison - These were taken from the street peering into an eatery. Here I like the 70 better. I don't know how to explain it but I just like how it is a little darker and it isn't as warm. It seems to me the 70 is a little cooler. For these types of portraits going into the shops I think I'll go with the 70. Note: I did have to take a couple steps back to get the same kind of framing for the 77.
70mm
77mm
I also noticed something else. Pretty much for all my shots the camera was making used f2.5 with the 77mm and f2.4 with the 70mm. The 70mm pretty much always used 800iso and the 77mm used 200/400/800 inconsistently. Unfortunately I only had 2 hours to work with and I wish I had more time to play around with the lenses and camera settings. I think the next time they do the rentals (in the summer I think) I'll definitely know what to do for better testing. Hope this helps.
(Apologies for such a big post. Is there a way to format the pictures side by side?)