The ArcadeVGA will make it easy to get the 15kHz modes both monitors can use, and the D9200 utility lets you use the 31kHz mode too. However, you'll still need Advance Mame for the 24kHz to look perfect, but most with the ArcadeVga and tri-sync monitors use 640x480 for 24kHz/Medium res games and turn on hardware stretching.
It's really up to you, if you're only going for 15kHz modes, the ArcadeVGA will get you there quickly and easily. If you're going to configure Advance Mame anyway to get the 24kHz modes, why bother with the ArcadeVGA's expense?
If you don't know, just get the ArcadeVGA, then if you decide you need 24kHz, you can setup AdvanceMame then.