If you can handle playing games in scanlines then go for the monitor. If you plan on playing a lot of Centipede all day then you won't notice. Overall, bigger is better if you're playing a lot of games like Donkey Kong and Pacman. Otherwise, it may be small, but the Eygo is the way to go. If you would rather kick someone's bootay in say...Street Fighter EX 3, then the scanlines are a little annoying. The games still look good on a monitor, they just don't look AS good as they do on an arcade monitor. Beauty is in the eye of the beholder though. I like redheads, you may like blondes.
Another thing to think about is the overall value. Down the line if you want to cash it in and can't sell it as a mame cabinet, you may be able to sell the computer inside, and you'll need a monitor to do that.