The 25" is the only way to go! Sure, its a few inches wider and deeper and heavier, but...
Control panel... sure, there's enough room on the 19", but I don't like being up on my buddies jock either while I'm trying to maneuver! That few extra inches is nice especially with double joysticks your arms kinda flail around sometimes. Its like playing Mario Bros. on a conversion cab... sure its only a few inches but DAMN night and day difference!
Monitor... The way the dedicated Smash TV 25" cabinets are made the monitor sits back and at more of an angle, so it is actually VERY comfortable to play long sessions as opposed to a 19" monitor thats right in my face and my neck is bent straight forward to see it. You can see all the enemies just fine both sides of the room and kitty corner, the angle is so good on the monitor it doesn't screw up the playability of the larger monitor.
Graphics... The game was made to run on a 25" game! You don't see the stretched out pixelated graphics of older games on larger monitors. Everything is colorful, sharp and smooth.
Cabinet... Who wants a game that looks like every other Williams cabinet made for almost 10 years? Gotta love that bitchin' shape and extra couple of inches of artwork on the big bad boy cab.
The main advantage I can think of with the 19" is SPACE/SQUARE FOOTAGE, I can think of no other reason I'd want one other than maybe it has a cool looking bezel, which could easily be made something similar on the 25" cab.