The noticable effects of lag will depend on the types of games you play and your personal level of sensitivity. For example, you're more likely to notice lag in games that require "twitch" skill or timing, especially if you have fast reaction speed or you're very good at the game (and therefore have developed a distinct "feel" for it). Input lag below 2f is almost completely irrelevant in, for instance, RTS games. Also, internet games like MMOs typically have so much connection latency that display lag becomes insignificant - this is further emphasized by the fact that these games are actually designed with lag in mind, so timing is never very strict to begin with. That's not to say faster displays don't help - obviously, being able to react 20ms faster is still a (small) advantage whether you need it or not - just that it's probably not worth sacrificing image quality over.
If you're an average gamer, I think you should be okay with the 24"ers. But again, if you're going to get the BenQ, I'd try to find the WZ anyway, since it's 10ms faster without any technical disadvantages (that I am aware of).