There are only 2 versions of Metro.
Metro (I'll call this one full) and Metro SE (limited to 4 audio tracks, 4 plug-ins at once.....you can read about them on the product pages).
Metro full comes in 2 varieties which are G4 and non-G4.
If you purchase full you get access to both versions........so if you are getting a powerbook you'd download the G4 and if it were an iBook you'd download the non-G4 version.
All other options on the store page are like upgrades from Cakewalk Metro or Metro SE to Full.
The real question that needs answering is what do you want to do with Metro? If you're only needing a few tracks of audio playing at the same time then Metro SE would be fine.
Also there is no penalty for buying SE and then upgrading to Metro full later.
SE is $70. Upgrading to Full from SE would be $250. Total $320.
Buying Metro Full straight away is $320.
(actually the real prices are one cent less, so if you upgrade SE to full you save 1 cent!!!
).
You could decide to start with a mini studio setup with SE and then later when you become familar with the program and want the extra features in the full version, happily upgrade it.