Someone asked a similar question about RasPi versus BeagleBone Black on this thread, so I'll adapt the answer I gave there:
Your question brings to mind a favorite scene in Alice in Wonderland:
"Cheshire Puss," [Alice] began, rather timidly... "Would you tell me, please, which way I ought to go from here?"
"That depends a good deal on where you want to get to," said the Cat.
"I don't much care where--" said Alice.
"Then it doesn't matter which way you go," said the Cat.
The same is true with development boards. Arduino, Raspberry Pi, BeagleBone Black, ST Discovery, PSoC 4 Pioneer and others all have things they do better than the rest, so it depends on what you want to explore.
One thing I would strongly recommend to anyone considering any community-supported development board is to first spend some time at the forums available for that board. See how people are using the board and what best matches your interests. See how active the forum is, and whether people are getting helpful responses to problems or are being told to RTFM (without providing link to said manual), or worse, "Google it" as if a newbie can tell which few of the 1000 Google results are actually useful. If you have questions, these are the people who are going to help you -- or not. Find out if people are having issues with the board and whether you want to face those same issues. Also, check out the wiki for the board. It will tell you if people are interested enough in the board to build up a good wiki and are therefore more likely to be interested in helping you when you get in trouble.
