It would help if you'd provide model numbers that we could use to look up specifications. That or links to the specific models you're looking for.
But, based on the information that you've provided, the 7" monitor appears to have more pixels, hence is capable of a higher resolution than the 8" monitor. that's not necessarily better, really. After all, how high a resolution are you going to tun on a 7" screen? 1024x768? I serously doubt it, as test would be far too small to read without making some serious changes to Windoze appearance settings.
Another thing to consider is whether or not these screens have a VGA input. VGA input is different from VGA resolution. VGA input means it has the 15-pin D-SUB connector that you have on your monitor at home for video. For a PC, that's the [/i]ideal[/i] video connection to your screen.
With most 7" and 8" screens, they are the same size horizontally, but the 8" screen is taller to make it a true 4:3 aspect ratio, while the 7" screens are shorter to produce a 16:9 aspect ratio.