I haven't played GMT's stuff but am currently engaged in a campaign of GB2.
Here a few comments re: GB2
GBII has a scale of 5 miles/hex
Infantry units are divisions generally 2-4 steps but can be "broken down" into individual regiments.
Armored and motorized divisions are made up of individual regiments and battalions.
Turns represent one half of a week.
OCS has an enemy reaction phase after player movement phase in which "reserve" marked units can move and fight.It also has an exploitation phase where reserved marked units can move and fight again. Basically ZOC's are rather weak verese leg ans Track movement (strong vs. truck though)
There is also a unique initiative sequence as to who is the first player each turn. Many experienced OCS-ers choose to go second setting up a deadly "double turn" called the "flipflop" which creates some tense moments to say the least.
Also GB2 has an interesting air system where air units can be used in ground support, interdiction, fighter sweeps, patrol, barrage vs. facilities and on and on. Each aircounter is a squadron of a specific type of aircraft, catagorized by fighters, tactical bombers, strategic bombers and transports.
A great game in a great system.