Ideally, just do what Cranky said. I play old games all the time like that.
If you are getting one of the two, there are a lot of variables to take into account. If the CRT is used, it may be pretty worn-out. CRTs, much like first-generation plasmas, suffer from phosphor aging over time which will reduce the quality of your picture significantly. You can even get an image permanently etched into the screen. Over time, an LCD may be better. You may also have to fix the shape of the image sent to the screen and weird stuff like that with a CRT. But if we're talking brand-new to brand-new, the better response time, lack of input lag, possibility for higher refresh rate and overall better picture quality of a good CRT could make it the winner. I don't think there are any good/high-end 4:3 LCDs anyway.
But really, if you already have a screen then just setting it to maintain aspect ratio is probably your best bet.