Pentax 16-50mm f/2.8 SDM will be more suitable than 17-70mm f4.
There is a couple reason into it. First you're right, it is about f/2.8, which is better in low light condition, or it allows u to shoot with lower ISO for better image quality.
The second reason will be the built quality. Many times in wedding photography, u will be in pretty tight space, either because of the small room or crowds around you. Often, your lenses will hit something. With built quality of 16-50mm, it will survive longer.
Other Pentax pro grade lens that I recommend to complement this lens is DA* 50-135mm f/2.8
To answer your question why Pentax seems to have a weird zoom length.. the answer is actually they are very rational in the lens focal length decisions.
When Pentax goes into digital era. they decided to stick with APS-C sensor (1.5x crop factor) and then invested quite heavily in making DA, DA Limited and DA* lenses which is optimized for their digital SLR cameras.
Because they don't have any idea or don't decide on making full frame (perhaps in the future or not at all). They try to mimic the focal length that are popular in film era.
For example, 16-50mm will closely 24-75mm
50-135 will closely equivalent to 75-200mm
On the other hand, Canon is planning to have full frame camera out, so they are releasing lenses for that purpose. So called "L" lenses.
16-35mm f/2.8, 24-70mm f/2.8, 70-200mm f/2.8, 24-105mm f/4 and so on.
Those lenses inevitably become very popular among crop sensor camera users in the last 4-5 years.
Now that many photographers are getting used to Canon or Nikon lenses, Pentax decision on focal length might seems weird, but it is not.
I wrote Canon vs Pentax
lenses at my blog a while ago, probably this will be helpful for you to check out what Pentax has available especially in high-pro grade prime and pro grade zoom area.