Before you read my message, please consider the following:
1) I've only shot two weddings in my life (both as a favor, and both this past month, as it happens);
2) I don't intend on shooting another one (things went well, but I have no desire to get into this sort of thing seriously);
3) they were both shot with Nikon equipment (which a friend - the father of the bride in the 1st wedding - lent to me for a month before the first wedding so I could get accustomed to it);
I used a D300 with a Nikkor 17-55/2.8 zoom almost exclusively on both occasions. I felt the zoom was most useful during the ceremony proper, because things happen very quickly then and you will want/need different framings in a short time span. I would not have been comfortable changing lenses.
The zoom was also very useful for the "candid" shots of the groom and bride's preparation and dressing up. Again, this happen quickly and different angles of view are called for in a short time.
Primes would have been fine for the "formal" & family pictures, but even then time is of the essence because everyone just wants to go to the party by then
I feel f/2.8 is preferable to f/4 because it gives you more possibilities to use shallow DOF for more "artsy" shots. Mind you, I spend most of my time between f/4 and f/5.6, but f/2.8 was used several times.
So my advice is: get a f/2.8 zoom. Two cheaper alternatives to the DA*16-50 are the Tamron 17-50/2.8 and the Sigma 18-50/2.8. I have no experience with them at all, but they are generall well-spoken of.
Other thoughts & notes not directly related to your enquiry:
My "base" settings on the D300 were M-mode, ISO 800, f/4 to f/5.6, 1/80-1/100s. The flash was in "TTL-BL" mode ("balanced-light", very nifty - I don't believe Pentax has an equivalent more unfortunately) with a Gary Fong Lightsphere Universal II (no dome, pointed at the ceiling).
I went to the church on two occasions before the wedding to perform lighting tests. I also printed some tests shots in 8x12" to make sure the ISO800 results were good when printed at that size.
For the first wedding, I had the D300 + 17-55/2.8 zoom as my main camera, with a D200 + 85/1.8 prime as a backup/longer reach solution. A bit cumbersome to carry around my neck, but not as bad as I anticipated. I wanted a longer lens because I felt 55mm would not be long enough for some shots.
I ended up not using the D200+85mm combo much, and was generally dissatisfied with the results it gave so I did not use it for the 2nd wedding. As it happens, lack of stabilisation was a major factor in the so-so results I got, along with the fact the I was limited to ISO 400 for IQ reasons. I should have gone with my initial idea and carry the K10D+FA77 for a "longer reach" body, and keep the D200 strictly as a backup.
So I guess my recommendation is a xx-50/2.8 zoom on your K20D, and a prime between 70 and 85mm on your K100D, both cameras around you neck at least during the ceremony. If you can, get a D-BG2 battery grip for your K20D, because you'll be shooting a lot of shots in "portrait" orientation, and you'll need 2 batteries for the whole day anyway.