Hi calamity,
I think I know why there's a big difference now. Previously I had soft 15hz installed as well, which I know now gave quickres more options to pick from when selecting a screen (Not all safe I know lol) resolution to go with a chosen game.
Now the game I used to select first was WWF Wrestlemania, this at the time, chose a screen mode which outputed this ultra sharp screen mode, and then this mode would stay in place for nearly all the other games I booted afterwards. Now the look this mode gave, mimicked
the look from an arcade monitor exactly, basically everything on screen was pin sharp and the screen kinda just (very vibrant image) glowed, it's difficult to explain.
The worse part is, I never made a note on what screen mode switch res had selected, being the complete noob that I'am.
I've only started to learn and compile notes on which screen mode, switch res now selects. These modes differ vastly to which monitor option you've selected through Vm maker to begin with. Also, these modes differ again if soft 15hz is installed.
I belive the mode it chose was an interlaced mode, as there was on screen flicker. I know a lot of people hate any on screen flicker, but my take though, is the slight on screen flicker just matches what you used to get in the arcades.
I've now played around with various different combinations, but the screen has never switched back to this mode
The one weird thing I remember though, is if I loaded up groovymame and say chose to boot Mk2 first, the screen mode it selected was more plain. If I then booted up wrestlemania, the screen mode I loved would kick in, and then stay in place if I came out and rebooted Mk2.
This I found weird though, as these games original resolutions are the same, so you'd think it would have selected the same mode.
Since my re-install, they do now share the same screen mode, but not the mode from before

Oh!! I've not played around in the mame (apart from monitor type) ini file, just the different options in Vm maker and with soft 15 on or off.