I've got both. I think for dodgy knees (plus additionally in my case a knackered back), the bow balances are better, and I like their funky looks too. The system 4's are also very good and are more conventional looking, but IMHO they are not quite as shock absorbing as the bow balances.
Throwing another one into the mix, I have these http://***********.com/sport_classic.html, which are great for my 'broken one too many times' ankles as I can set them individually to the best angle for each ankle. I also have a bolted back together broken hip which they help with. My knees are about the only part of my legs that aren't b*ggered though so I can't comment on knees specifically. Also I got the metal foot so I could get a wider foot area but it can lead to rather cold feet in winter.
I looked at the Bow Balance as my knees and ankles were painful even after a short ride with normal stirrups but I found they were very heavy and I ended up buying the Avacallo Opera stirrups which are composite with a very wide tread. They solved all my pain issues on the very first ride and I was so impressed with them two other people on the yard have also got themselves some.
I got the bow balance because my ankles were killing me after riding! They've absolutely sorted it. Love them! Worth looking at other manufacturers/recommendations too as they are sodding expensive!!