ePub is an open standard. It isn't owned by any company. There are varying forms of DRM for ePub, with three main ones. Adobe's ADEPT, which is what you'll find at almost every place that sells DRM'ed ePubs. Next up is B&N's DRM used on the nook. Newer versions of ADEPT's SDK does support B&N's DRM, but, some device manufacturers use older versions, so your milage may vary. Finally, is Apple's iBooks. iBooks can only be bought via iTunes on your computer, or via the iBooks app on an iOS device (iPod Touch, iPad, and iPhone), and only be read on an iOS device.
So, DRM free ePub is an open standard, but 3 main forms of DRM are used with ePub. One is extremely widespread, one is only from one source but can be used on some other devices, and one extremely limited one. Basically, can't really go wrong by going ePub.
Personally, I would recommend Sony or nook, for readers.
nook is from Barnes and Noble, so supports their proprietary DRM, as well as the Adobe DRM. On a stock nook Simple Touch, nook Color, or nook Tablet, there is a limit on how much you can "Sideload". Sideloading is when you transfer a book from your computer, rather than downloading directly from the built in store on the device. For the built in storage, you can only sideload 256mb on the Simple Touch (of the 2 gigs storage), and 1gig on the Color or Tablet (of the available 8 or 16 gigs). That is still a lot of space, but that limit can be changed if you root the tablet.
I personally have an older Sony PRS-505, and my wife has a PRS-300. Both have long since discontinued, but are still really good readers. The newer PRS-T2 looks like a really great tablet, and I am tempted to get one. No restriction on sideloading like the nook has (all internal memory available can be filled with sideloaded books).
Both Sony and B&N now use Android as a basis for their readers. You can root them and install other software if you want. Kind of funny seeing Angry Birds being played on an eInk ebook reader.
PS, I know that I did not capitalize the "n" in nook. Blame B&N, they purposely stylized it that way.