Older thread but I thought I'd kick in my experience. I got first the POC VPD 2.0 knee only (not long), then the TLD KG 5400.
The POC feels like it offers a lot of protection, but it's very heavy, very hot, and irritated the back of my knees. Even sliding them down to my ankles for uphills, the weight was noticeable spinning. Still, they feel pretty burly, and I'd definitely lug them along if I go ride something hairy. They did stretch out a moderate amount.
The TLD feels like it offers much more minimal protection (especially on the side), but the frontal is adequate for normal stuff. I went over my handlebars on a curb with them (don't ask), and while I got a bit of a raspberry on the side of my knee from sliding, there was no bruise at all on the front where I impacted. They seem to breath pretty well, as I've worn them on several rides in 80+ degrees and don't bother to slide them down until my final climb out. They're so lightweight that I don't notice them on my legs when I do slide them down... which is nice, since it saves the hassle of pulling them off and stowing them in my pack. They do stretch out a lot though, so make sure you follow the size guide on the TLD website (it suggested a small, while I wore a medium in the POCs - I'm 5'10" 155#). My biggest complaint with them is that the plastic strip that covers the top of your shinbone creaks when it flexes until you break it in.