I can't speak with any authority on the HK Hardline Playing trousers, but when I looked at them online, they seem to be well padded, and the reviews all look good. I got lucky and managed to pick up a pair of the Empire Playing Trousers, from Ebay. The difference from playing in my jeans was great, there is a lot more air flow so it helps keep you cooler and the padding meant that I was hit on the knees a couple of times, and the paintballs bounced, and the padded knee's also make kneeling down really comfortable,the HK hardline trousers advertise the same features as the Empire ones, so in my opinion there is not alot in it.
It would be interesting if any of the more seasoned players than me, could comment on is there a great deal of difference between the brands?