omg, I visit neo geo forums daily for almost a year and I didn't even know there was a shootemup section - my favourite genre...
I use a monitor in TATE with a scandoubler to allow my GC and PS2 to use it (although the GC I only have Ikaruga). My DC plugs in directly though ofcourse on the VGA input of the scandoubler so switching between all machines is really convenient.
I have a cabinet, but it is horizontal, thought about a vertical one, maybe oneday...
I don't like turning the 20" IBM monitor back and forth all the time though cause it's too heavy. I looked hard for a swivel LCD screen and guess what? They make them, but they all rotate right instead of left