You will get a better response time usually with a 17 inch but also, you will get a sharper picture too, as the majority of 19 inchers run at 1280X1024. Same amount of pixels in a smaller space, equals a sharper pic. Old rules still apply though. If you want a fast screen for games, then get a CRT monitor instead.
Last week, I bought a 19 inch LCD TV/Monitor and although it works beautifully at the above resolution, I prefer my 17 inch Iiyama LCD for detailed photo work. A bit ironic really - I bought the 19 inch to save some space (and let me watch the Cricket!) but now have ended up using more space with the two monitors side by side. That is progress, allegedly!
Conan