Still trademark pending, the "3 mech advantage" is commonly the discussion between Prem. vs. Pro. Usually a major mech, a mid mech, and a minor mech are the GAMEPLAY differences. Examples...
GZ - Building, Mecha, Bridge
TWD - Crossbow, Bicycle Girl Ramp, 2nd Magnet
STH - Projector, TK Lock, More animated DG
SW - Hyperspace, Death Star, Escape
Venom - 3-way ramp shot, Doppleganger, jump ramp
Here you weigh price adder vs. how much those 3 things mean to you. Since the mechs are usually spread out, the Prem usually feels more "alive" as well. But this is where the debate is usually 50-50% Pro vs. Prem.
Then you have games where the Prem doesn't really hit the "3 mech advantage".
DP - right orbit ramp is neat but not "wow", drops instead of standups is not worth a Prem upgrade.
IMDN - The SC Door/Mummy lock is cool but not necessary, same with the Underworld ramp.
AIQ - Rising disc is cool but kills flow, Captain Marvel Ramp was neat but rejects. Pro was a rock solid flow beast.
TMNT - Glider (meh), van lock (double meh)
GOTG - Game sucks anyways, so why spend extra on obtrusive arms and 2 unnecessary magnets?
In this case you often have the "Pro is more than fine" crowd vs the "I buy all Prem's so I'm going to defend it" crowd. This is more of a 70-30% split for Pro's. The Prem is still a more feature-rich game, and is still a lot of fun, but the Pro has a stronger leg to stand on since more people don't see the value in the upgrade.
Each release gets this sort of analysis, more or less. All comes down to those 3 mechs and how good they are.