I was surprised to see that the difference in size between the two lenses isn't that significant. The 17-50 2.8 is probably a bit heavier though.
See size comparison:
Compact Camera Meter
With that in mind, and with the prices that these lenses are selling for nowadays (very affordable!), I think the newer 17-50 is almost a no-brainer.
(For disclosure, I don't have either lens. I thought about it, but I love my Pentax DA 16-45mm f4, and f4 is fast enough for what I use it for - and I much prefer the colors I get from Pentax lenses. If I were to buy a constant f2.8 lens, I think I personally would look at the Tamron 17-50 2.8, because I really like the look of the pictures taken with it)