They really are quite different.
The Grippapro is the same as many supplied units, i.e. a steel frame over a standard Wydale tank and as such has a small 5-10mm gap between the tank and frame on all sides and is bolted through the floor with spreader plates.
The GrippaMax is quite a different beast altogether, although the baffled (it's not honeycomb just plastic sheets bolted together in circles) tank is provided by Wydale it is a custom produced unit held in place by two very substantial bars front and rear and two very substantial (get the idea) steel straps running front to back and integral to the moulded tank. The end result is a tank held firmly in place on all sides without any gaps. This is then bolted and bonded (strong adhesive) to the floor with spreader plates.
If you want a professionally engineered crash tested system go for the GippaMax, to be honest I don't think there's a great deal of difference in the price.
I actually have both units sitting in my garage.
Hope this helps