Get both, stack them if you have to.
Why do you think the 17" is limited to 240p? That doesn't sound right, you might be surprised once you feed it a signal.
Make or buy a VGA-SCART cable and play Groovymame. There are a few options out there, but if you're handy with a soldering iron you can make a decent cable from an old VGA cable and a SCART header you can buy online for a couple of bucks. I get them in packs of ten. There's a thread pinned on the Monitor/Video forum that discusses it, though not really active anymore.