Note that for shmups on mame there is an active project on shmups.com re a special recompile of mame to better emulate shmups (i.e., reduce the lag that's currently in mame).
To nitpick, I want to point out that ShmupMAME opts for less accurate emulation. MAME emulates lag present in the original games, but most modern PC users have some lag due to their displays or display options that will stack on top of this. ShmupMAME bypasses the original game's lag in order to compensate for the user's laggy display pipeline. Assuming nothing has changed in the past couple years, at least.
Basically, if you have a gsync monitor and want an accurate experience, use standard MAME over ShmupMAME.