Which Windows 7 to install?

King Arthur said:

If you require or want absolutely everything Windows 7 has to offer, go with Windows 7 Ultimate.
If you are a professional, power user, enthusiast, or basically do more stuff than just browse Facebook and Youtube on your computer, and don't require or want Windows 7 Ultimate, Windows 7 Professional might suit you better than Windows 7 Home Premium.
If you are just a casual computer user or otherwise don't intend on using your computer for anything other than very simple everyday tasks like Facebook, Youtube, and emails, Windows 7 Home Premium will be just fine.

As for me personally, I use Windows 7 Ultimate on my desktop and Windows 7 Professional on my laptop because I wanted everything Windows 7 had to offer for my desktop and Windows 7 Home Premium was too limited for me.

Just out of curiosity, what features did you need from Ultimate?

More than 16GB of RAM (with X64)? Two physical CPU sockets? Join a domain? Bitlocker? Language packs?

If memory serves, Pro doesn't support Bitlocker or the language packs.

There is supposed to be no performance difference between the three versions, for hardware configurations that are supported by all three. (16GB of RAM or less, usually. Few home users have dual CPU sockets.)

From Ultimate: Japanese language pack, BitLocker.
From Professional: 16GB+ RAM, better system management, better networking management, XP Mode.

Besides XP Mode, which I'm utilizing in two copies (ENG and JPN XP Mode VMs in VirtualBox), and stuff related to managing the computer and any associated networks, all of the rest are options that I wanted available to me but not necessarily stuff I need right now. I've always went for the biggest thing there is when dealing with tiered products (unless the biggest was simply ludicrous), and besides the initial up front cost I haven't gone wrong.

Of course, this isn't an option for everyone, so I do strongly advise everyone looking to buy Windows 7 to properly do their research before investing their money!