Yes I agree with IL; SPA (Self Propelled Artillery) is the term given to things like the Hummel, Wespe, Bison etc. AG (Assault Gun) is for things like the STuG III, Sturmtiger, sIG 33B etc. And obviously, TD (Tank Destroyers) is for things like the Nashorn, Jagdpanther, Marder, Hetzer etc. And I suppose that Self Propelled Gun is a common collective term for all these different classes/variations.
I agree that we do not need this level of discrimination between the classes of Armoured units for the Global or indeed Theatre level games. However, if FMG are only producing these sets to replace the WOTC units, then yes, it would be great to have two types of Tank for each nation. Italy has a light Tank and a SPG so far. I for one, intend to use these FMG pieces not just for A&A, but also for other games (including my own) and it would be fantastic to have SPGs on a Tactical or even Operational level. This is my opinion from a historical standpoint.
Also, from an aesthetic stanpoint, I am somewhat scratching my head over the sight of SPGs on the Global and Theatre level. It just doesn’t look right, as these uinits were normally a Brigade-sized attachment to the Armoured Divisions. However, if we do indeed get 6 Tanks and 6 SPGs for Italy, then I can’t see it being too much of a problem as I will use the Tanks first, and only use SPGs if I have to. :-)