I've heard that the HD Collection was glitchy mainly (and ironically) for PS3 folks. Being a 360 user, I haven't experienced much in the way of glitches, per say. I have come across audio looping, but that's not really an end of the world kind of thing. About the only thing really that upset me with the collection was the changing/removal of certain sounds present in the original PS2 version. For instance, veteran players no doubt knowing, the certain boat horn noise that usually plays when going down a certain looooooong staircase (don't want to spoil for newbies) is no longer there. There's only a bizarre clicking noise amongst silence. Being one of my favorite sounds in the game, that really ticked me off, and I personally don't see Konami planning to fix any of that. If they can't be assed to pay the original voice actors, I doubt they'll commit much more to the gamers.
But anyway, to the initial question at hand, I would personally choose the HD collection over Downpour. In my honest opinion, every Silent Hill game after 4 has pretty much sucked and only diluted the quality of the series. If you want to get the true feeling of Silent Hill, I would DEFINITELY recommend the Collection, hands down.