A lot depends what stage of tune you are aiming for really?
If you are starting with nothing then a level 8 would be the best one for you to get as it has a greater capacity for future mods! Although a Level 6 (standard in a 2wd Saph) would be adequate.

Level 1 - small yellow mixture screw and brackets
Level 6 - larger white mixture screw and brackets
Level 8 - white mixture screw and no brackets 2wd = red label
Level 8 - as above 4wd = Green label

The above is the general way to tell the difference but a check of the numbers on the covers would also be a good idea!
HTH
Spadge...