The OEM copy you are required to provide all tech support for yourself. The Upgrade copy is considered retail, which means MS will provide some support (useful if you lose the disk), plus yes the benefit of being able to move it to new computers.
However being an upgrade you need to already have xp or vista on the machine you are going to move it to. You mentioned already having XP and Vista. If they are OEM copies unfortunately they are locked to the machine they are currently on so having a retail upgrade copy of Win7 doesn't do you much good (as far as moving from machine to machine). However if your XP and Vista are retail (or if one of them is) you can move it to a new machine along with moving your Win7 upgrade to a new machine.
Basically it comes down to: Retail XP (or Vista) + Win7 upgrade = Retail Win7
At least for the purposed of moving it to a new machine later.