The new MacBook Airs come stock with Intel’s Core i5 processors. Speed tests show that the i5 is about twice as fast as the Core 2 Duo found in the previous MacBook Airs.

But if you’ve specced one out online, then you may be wondering what’s with the i7 option. You can pay $100 to upgrade to a 1.8GHz Core i7 processor. However, Apple doesn’t say much about the difference between the two other than the negligible speed bump and the slight increase in L3 cache.

So, what is the difference between a MacBook Air with a Core-Duo i7 rather than an i5? Actually, not much.

There used to be a big difference between the i7 and the i5, related to Hyper-threading — the i7 had it enabled and the i5 did not. Hyper-threading means that though the Dual-Core i7 is physically a 2-core processor, virtually it can act as a 4-core processor (assuming the software can take advantage of the virtual cores by multithreading).

But for the new mobile Sandy Bridge processors that the MacBook Airs are using, the i5 now has Hyper-threading enabled. This is great news and it means that the i5 MacBook Air is comparable to the i7.

But that doesn’t answer if $100 is worth it for the i7. If you plan on using your MacBook Air for several years and don’t mind spending an extra C-Note, then I do think the i7 is worth the money. I went with the i7 option but only because the local Apple Store had them in stock.

If you don’t know or don’t care about the i7, then you won’t be missing out if you save your money or buy a nice laptop case instead.