Well, here is how I see it, but bear in mind, im still a CRT man and am by no means an expert.
The 19" monitor has about 15% less brightness but about 10% more contrast. It also has a response rate that is nearly half that of the 17". The way I understand it, the brightness level will only be noticeable if you have a high amount of background lighting in the room, but assuming this is a gaming machine, you probably have it set up in a nice area to avoid light anyways. The lower response time is probably the most crucial for high frame rate gaming. So in my opinion, the 19" is the one to go with.
Were I you, Id do some research on the web about brightness ratings etc.... Good luck!