What Mohawk Dave said above ^, but ...
While the 12 point Midgets are just a shorter wrench than the 'short pattern', but cover a similar size range, the 6 point Midgets cover a much smaller range of sizes. You might term these 'ignition' wrenches!.
Also bear in mind that Snap On wrenches are longer than some other makers (such as MAC). So a Snap On 'Standard' equates to a Mac 'long', a Snap On 'Short' equates to a Mac 'Standard', and a Snap On 'Midget' to a Mac 'Short' (more or less)!
Which to get first?
In all honesty, get the Standard first. Most times when you need a wrench, you need the leverage. After that, get the Short. They will get in to places the Standard sometimes cannot, but are still a usable length!
I personally wouldn't bother with the Midget, other than in the really small sizes, as they just don't have enough leverage to slacken the average fastener. There may, however, be places that only a Midget will fit!
I have some short length ring wrenches (box end to you guys) that I use far more often than Short pattern combo's! Were I buying again, I'd buy the box end wrenches in standard and short patterns, and probably the combos in standard only. After that I'd get stuff like Flare Nut Wrenches (the Snappys are uber useful) before the short combo's!