Well the box that retails for 325 doesn't look that playable to me. It's got about the same number of stealth suits as it does fire warriors, and 5 tanks (unsure of what kind of tanks). Plus it has no kroot.
That one seems to be more for apocalypse rather than regular gaming. If you plan on eventually buying all that stuff then go ahead, but you'll need to buy other stuff (crisis suits, broadsides, pathfinders, kroot) to make it effective.
The one that retails for 90 is meant to be more of a ready to go 500ish point army (I think, I'm not a Tau player). It's already like getting a free unit in this box, plus you can get another 20%-ish off by buying from a online discount store.