I never had 70 (used to have 40 though) but used to own 55. From what I've seen/read I'd say this:
- Build quality is a toss. You get high quality WR plastic lens or metal. If anything the 55 will have bigger chance of surviving knocks and less chance of developing wobbles. Also don't underestimate the WR aspect.
- rendering? Well the DA* is quite similar to FA43 while I'd imagine 70 would be similar to 40. For me that meant I liked 55 colours and contrast more. DA ltds felt bit cold and surgical to me. Having said that shooting wide open you might want to be careful with backgrounds with DA*. Some say it's out of focus areas have charcter, I found them distracting at times.
-size matters! Where you can take 55 you would be easily be able to take 40+70+one more small lens. It's up to you whether you prefer the FOV from 55 over having one shortr one longer lens combination. Personally I had no problems using 55 as the only lens for some walks and if I wanted wider I'd taken FA*24 so I had the 35-85mm equivalents covered.
-AF, if 70 is anything like 40 it'll leave 55 in dust. For distances over 3 meters the 55 does OK (also depends on the body) but for closer distances you might struggle. I used the 55 on K-7 and taking pictures of people at close range I found the combo simply too slow, many nice moments got lost or got captured slightly OOF.
- me? If I ever will go back to Pentax my 1st 50ish choice would probably be 50/1.7 F or FA. Followed by FA43 and K50/1.2 (depends how bad would my eyesight get). Just then it's DA*55 and F/FA50/1.4. After having tried the dark side I can't imagine getting the keepers and keeper ratio I have now with Da*55...
Having said that, it still took some of my finest images, but truth to be said, my son was few months old and didn't move so much back then. The moment he started to do so the keeper ratio with DA*55 when used in the room at close range dropped off significantly...
Good luck choosing