I can't speak to the Kreiga, but I've had the Wolfman Peak tailbag for over a year. Used to carry it on my WR250R and have only had it on the S-10 a couple of times. Don't need it for commuting duties and it's not big enough for longer trips. It's a well made, sturdy product as expected from Wolfman. My only gripe about it is that cranking down on the mounting straps deforms the bag, though it's flexible enough to still work while slightly deformed.
Functionally, I find the zipper top of the Peak more convenient to use than the roll top of the Kriega. Your need for waterproofness may outweigh convenience. On the WR, I now use the Wolfman Explorer saddle bags instead of the Peak tailbag and appreciate not having to do a high-Karate kick to mount the bike. I can still use the Peak, and stick it right on top of the Rotopax mount or the Rotopax can if it's in place.
In your price comparison, you didn't state which size Kriega bag (I presume the smaller). I would think the S-10 has ample room for the larger bag and would err on the side of more capacity.