A 720p panel would be a mistake if you intend to use effects like scanlines, the resolution is simply too low for that, so the ViewSonic would be better.
Now that ViewSonic like many monitors of the brand has the ability to run the game's native refresh rates (for instance a 54Hz game won't be forced to 60Hz), but that's under the condition you're using GroovyMAME and CRT_Emudriver, which means no R.Pi but a Windows PC with a compatible discrete AMD graphics card, plus some time to learn how to set this up)
Last thing; I don't know if this is still true today but I've been reading monitors reviews for many years and learned that 28" panels have pretty bad reputation in terms of quality and performance while 27" and 32" are okay. Though most people who aren't as nitpicking as I am over LCDs probably wouldn't tell the difference and be happy with a 28 anyway. ^^
EDIT maybe you can get the Pi to output the exact refreshes and work with that ViewSonic as well, but if that's any possible I don't know how, you'd have to ask Calamity the question at the GroovyMAME forum.