"Better" depends on what your workflow is and how you like to work.
For me (and others may vary), I have my image, with nothing else, on the main monitor. I have a Wacom tablet, too. On the other monitor, are all the other bits of PS or LR (LR has a built in 2 monitor mode). I use a lot of keyboard short cuts (I have a Logitech G15 and all the things I want to regularly do are just bound to the additional function keys - 1 press, no finger gymastics required).
I don't have to "jump" to the other screen, but I can see information there (OK - of course sometimes I need to jump to the other screen, but not very often).
So for me it gives me an image without clutter and it gives me the maximum real estate on that screen for my image. It also gives me maxium real estate on the other screen for all the other bits and pieces.
When it's not photo editing, it could be web page design where I have one screen showing me the current page and how it looks and one page with my CMS open or perhaps some code or whatever else I'm wanting to look at.
When I'm researching something, I can have my document/slash notes open on one page and research material or website or whatever on the other.
The list goes on.
The advantage of two different screens is the ability to have applications automatically open on one or the other and to have the screen at the side slightly angled and to provide a visual reference break between the two (or at times three, actually) which helps with organisation.
Personally, I don't want to be flipping between things - it takes time and concentration (or breaks concentration) and detracts from the task at hand. One really large screen can work - Eizo actually make a great screen slicer application that lets you treat it like multiple screens (which helps with window placement and focus control and other things), but it usually costs more for the real estate and you tend to get extra height that you don't always want relative to the total width (for some, of course, the height is great). The other advantage of multiple screens (which relates to the point I just made) is that you dont' have to have the primary one in a landscape orientation. If I'm working in portrait orientation or reading a document that's long or whatever it may be that benefits from portrait orientation, then I can just rotate it and use it that way, but keep the other monitor in landscape. You can't do that (have both at once) with a single monitor.
Oh, and whilst I said I don't like flipping between things - there can be an advantage and it's where I use a thing call Desktops (it's a tiny, tiny little Windows app from the Sys Internals guy - Mark Russinovich. This allows me to "flip" from two screens with PS to two screens with Bridge, or two screens with Directory Opus, so I can go from file management to photo management to photo editing at the press of a key and each time take advantage of the two screens configured as I prefer with working away.