I wouldn't go with those knee guards you linked. One strap won't hold it in place at all, you'll be fidgeting with them all day. I used the cheap $20/pair knee guards last year, shift was my favorite, but I also wore a pair of Fox guards. Broke four of them over the course of the season. They'll walk around a bit, I always had them slide down and outside the knee. Keep in mind they're made to be worn with motocross boots holding them in place.
I'm now wearing .... worth every damn penny. Put them on in the morning and they stay put all day; comfortable enough I don't realize they're on. A little more bulk to them than the cheapo knee guards, but the protection is way better and they still fit comfortably under my pants.
I disagree with DenverR1 about being difficult to put a knee on the seat with them. As mentioned ^, I don't even notice I have them on anymore, and when you smash your knee into the gas cap/cowl/rails/skis/windshield (i wreck a lot) you're not left with a huge bruise for the next week.
FWIW, I wear Klim adrenaline boots and the two don't interfere at all. They meet up nicely and there's no gap in protection between knee/shin guard and boot. Snowboard boots should be fine with just about any knee guard on the market.
