Which single board computer should I get if my purpose is Linux and programming?

If you only want to deal with software and not hardware then you would be better off dual booting your PC with Linux. Any single board solution like the Raspberry Pi or BeagleBoard will require you to do some hardware to accomplish things. I know you want it pocket sized but once you add a monitor, and keyboard and/or mouse it's no longer pocket sized. It may be best if you go with an already completed solution that won't need aditional hardwatre to run.

These small computers that you mention are all pretty great if you've got a scenario where you want computing power (say) in a low-power or mobile application, or where cost wouldn't justify a desktop machine. Using them may give you some ideas on where they could be applicable. Some examples: providing Internet connectivity to a kiosk or vending machine, or telemetry.

But coding and compiling on these computers is a slow process; better to write the code on a laptop and transfer the files (via the network connection) to the board when you want to test it out. Since most people who are developing software for such boards tend to already have a desktop/laptop, most comms is done via the network connection, thus saving you from having to plug in a monitor and keyboard to the device). So it's not compact, in practice you'll still need (well, highly highly desirable) your laptop when developing for these boards. Furthermore, you may wish to compile on your laptop first, to test it out before spending time compiling on the target board (note - there are ways to compile directly for the target board but on your laptop - this is rather complex although a web search may reveal if vendors or others have provided an environment for this to make it easier for you). And learning programming is far easier on a PC/laptop. The concepts are identical. You will use (say) gcc on Linux on your laptop, as you would on those boards.

For these reasons, as Michael says, better to use a PC and dual boot, and run Linux (or run it inside a VM - if you've got lots of RAM on your laptop and a modern CPU, the VM will run extremely fast). Are you learning programming? If so, it's worth buying a few books - sometimes it's easier to learn programming with a textbook as well as the Internet.

Having said all that, you may still wish to obtain a board, so you can simultaneously get an idea of it's capabilities, but be prepared to learn a little about hardware (initially not much). R-Pi has a big following so you may find it easier to initially learn on that platform, although some others are more powerful; it really depends on your final application requirements. The code you write will be portable to an extent (and very portable if you rely on Ethernet and serial for connecting to the outside world).

But the thing is, I already have linux dual booted on my laptop and my desktop and have played with Gentoo and whatever. I kinda want something for the sake of entertainment. I think it'd be really cool if I could have the board connect to my desktop but I think that will just be like ssh or ftp. Do you think I should get it just for the sake of entertainment?

So I should not get them if I'm just going to mess around with them like a computer? I'm kinda confused on its purpose I think.

And, yeah, I'm learning programming but I can just learn it on my PCs.

I kinda wanted to get it because the concept of a  credit card sized computer seemed pretty cool but I think I probably shoudln't if I don't have use for it.

Those boards are like over powered micro controllers or extreamly under powered linux based computers. They are designed for developing hardware and software. You decide the kind of hardware you want to interact with or controll then program it to use that hardware in the maner you want.

Athena C wrote:

 

I'm interested in Raspberry Pi, Beagle Board, and Odroid-U2 so far.

They all seem to run Linux. I'm more into software so the specs are a bit foreign to me.

Anyway, I plan to use it to mess around with Linux, programming and pretty much only software stuff.

I'm not too concern for price so long as I get the value worth the price.

I'll be happy with it so long as I can manipulate the OS, compile and run my cpp code, and carry it around in my pocket.

As Michael Pilcher says upstream, it's only the RasPi board that fits in your pocket.  Or, you can "roll it up in your T-shirt sleeve" and be "too cool to believe", to quote Jim Croce.  But if you really want a complete machine that fits in your pocket, you probably need to hack a smart phone or small tablet.

Here's my suggestion: start with the software.  Conceive the program you want to carry around in your pocket.  Get it running on your GNU/Linux PC.  By then it should be obvious which tiny board you want to use and what peripherals it will need.  Also, by then RasPi may have a tiny display that plugs into its DSI connector.  Or you can use a BeagleBone with a small LCD cape right now.

I know it's fun to acquire tiny boards and watch them blink lights, but in my experience it's usually the software that takes the time and effort in end-products.

JMO/YMMV