Not sure about quality I think there isn't much difference but for RGBHV-DB15 no additional boards are required. But you'll need to purchase a component if it' not included for the RGBHV-RGBHV via BNC's (depends where you buy)
I was looking at nexnix and im told the board included is the BNC component/composit/scideo board which allows me to do RGBHV in...... hence I have the choice of either DB15 or RGBHV in at no additional cost
I've used the RGB-DB15 input. The picture is clearer but you lose control over the colour and some programmes look awful with very high colour levels. I don't know if you lose the colour control via the BNC input but it may be worth enquiring.