Assuming you are talking about "MacBook Pro's" and "MacBook Air's"…there no need to cross-post. Questions about notebook computers don't belong in the desktop area.
As far as when to buy. This is up to you. We cannot predict the future…as to when & what Apple will release next (we don't know & Apple doesn't tell us). If you need a computer now…purchase a computer now. If you can wait…wait.
For me (if I was looking for a new computer). The "back to school" gift card deal is not enough incentive…if…I was concerned about what coming next.
Logically think about it from a business perspective. The computers that Apple sells at back to school time…are basically the…"Let's clear out the warehouse" stock.
Why? Lately Apple has been following a trend of releasing new notebook computers in the "Early" and "Late" parts of the year. This means something like February and October. So the computers that are sold at Back To School Time (and Christmas)…will in a very short time…be the older discontinued model.
So it comes down to…is the Back To School gift card promotion enough incentive to get a computer now…or better to wait until October when new models have a good chance of being released.
Like I mentioned above. For me…if I'm spending around $1500 +/- on a brand new computer…I want the latest & greatest. Unless…I'm getting a "killer" deal on a current model. And a $100 gift card (to me) is NOT a "killer deal". Especially since the gift card saves me nothing on the actual computer purchase.
- Nick