To some extent it depends how good your eyes are. I prefer a bigger picture (an effect of being > 50) and you can get a 19" TFT for as little as about £300, but I don't know how good they are.
If possible you should have a good look at both sizes in action running the usual sort of programmes that you use. Don't just try them for a minute or two, give them a good workout.
As for response times there are 19" TFT monitors with lower than 25 mS response times. The main problem that I have noticed with respect to response times is that as the time is reduced from 25 to 16 or 12 or even 8 mS the angle of view of the screen also reduces, both horizontal and vertical, from a possible 175 degrees towards 160 degrees. I think that this might be more annoying that a slower response time unless you are an avid games player.
All of the above is just my personal view and may well be different from that of other contributers.