A few things.
Listen to SailorSat (the creator of Soft15kHz). Great post above.
Soft15kHz with a compatible card is more powerful and flexible than an ArcadeVGA.
"GroovyMAME is smoother than Soft15kHz." NO! Not at all.
If you're getting tearing, you don't have MAME configured correctly. You need run the game at the refresh rate you're output, using the "sync to refresh" option and DirectDraw. Then adjust your refresh rate in Powerstrip to match that of the original game. This causes the game to run at it's original speed, frame per frame, completely smooth. Don't use triple buffering or wait vsync as they add lag.
If you're getting smooth scrolling with another method, you're likely adding lag, forcing the game to run slight off of it's original speed, skipping frames, or a combination of these. Convenient workarounds do not produce optimum results.