The ViewSonic. I am a huge fan of Flat Panels, but I can't stand Plasmas. they suck. If you are talking about a LCD TFT, then All i can say is get the sony hands down. but if the sony is a plasma don't touch it.
I prefer CRT to LCD, especially for gaming. While there may be a few (high priced) LCD that are almost on par with a CRT, nothing (ABSOLUTELY NOTHING) beats a CRT for gaming IMO.
I'm not quite sure how plasma stacks up against CRT, but from what I've seen it's just a faster LCD (but not as fast as CRT), hence the reason I prefer a nice CRT (with high refresh rate) to a low end (or even high end) plasma/LCD
I like flat pannel monitors cause they save so much space and weigh a lot less, but i don't think i would ever buy anything from sony unless i had too. So I guess i'd go with the viewsonic because it's still a decent monitor.
Like modemide said, if you play a lot of games then get the CRT, as they will display better and near infinite colors. Plasmas or LCD are good if you program or productivity, there faster but only have one native resolution and quite pricey if you go for plasma