Depends which model of monitor you are looking at. If it's being sold as a monitor then it should at least support 640x480 so you can see the boot screen. The only exception to this is that I'm aware of are certain models of the Apple Cinema Display, but there may be others, read the spec sheet for the model you are interested in.
Don't worry too much so long as it'll show 640x480 and it's native resolution, you can use the scaling built into your graphics card to acheive any other resolution, even non-standard ones like 960x600.
As for your original question, would you rather have a 16:9 monitor or a 16:10 monitor, the latter is squarer than the former and thus will result in small horizontal bars while watching widescreen films, with the trade off that when watching 4:3 films the vertical bars will be smaller.