First off, any such decision hinges primarily on the question 'what do you want to do with it?'. So before you dive in you should probably figure that one out, people on this board will be better able to give you advice. You say you are a beginner, but does this mean that you have absolutely zero experience? Do you know c/c++? Do you have some electrotechnical experience? What is your background?
Also, this is an Arduino board to naturally you will get many pro-Arduino opinions. Allow me to Provide you with my $0.02, as a software engineer with moderate experience in embedded systems. Arduino's are great as a cheap, standard platform. I use my Arduino Mega for just about anything that needs quick wiring up to a computer (wiimote, GPS receiver, stuff like that). I also use it as a primary target for testing generic embedded software (JVM for instance), because it's just extremely convenient and it has a metric fuckton of easily accesible I/O pins, as well as a nice range of shields (LCD being the most useful for me). Arduino comes with an IDE that allows you to program it, but I never use it, so I can't comment on how well it works. It has its own language, which some people like I guess, and a ton of libraries to drive all the peripherals. I suggest you have a look at it before you commit to buy. Personally I just write all my stuff in C which works just as well.
Now, talking processing cores, if you can program in C/C++ you aren't fixed to any platform as you can just use whatever chip you like as long as there's a decent compiler for it. In terms of compiler support you'll probably want to go for either ARM or AVR, as both have excellent and mature GCC ports. MSP430 has great electrical characteristics (power consumption) but lacks coherent GCC support (trust me on this, we are in a world of pain here with these new msp430x chips, and older models only go up to ~48-60k program flash depending on RAM size). Some like PIC18, but they are mostly EE guys who don't have to program the f*ckers, as there is no decent, free compiler for it and the official one is a joke (inb4 shitstorm from PIC18 fans:P).
So assuming you go for either ARM or AVR, there's a whole range of boards you can play with. Here's some I like:
The futurlec stamp boards are nice for breadboarding, but may require external programmers, so make sure you get those too if you're ordering. The LPCExpresso boards are a combination of a JTAG and MCU board that you can separate by cutting the PCB (nice). The teensy boards are really cool as they are USB slave devices, so you can program them to act as a HID device (i.e. joystick, keyboard, mouse), mass storage, etc.
Finally, the programming part. As I said, if you know C/C++ you can just pick any of these boards depending on their characteristics (speed, memory sizes, I/O, etc). Prepare to download and wade through datasheets though, to figure out how to drive the I/O pins and whatnot. For AVR there's avrlib (although the download seems to be broken, I have a copy somewhere if you want it). Arduino makes all of this simple for you by wrapping it in easy-to-use libraries, which makes it easier to get something simple working quickly.
My advice to the absolute n00b, assuming he/she wants to learn about embedded systems, is to get an Arduino. Use the Arduino IDE just to get your feet wet, try a couple of simple things like hooking up a potentiometer to a servo, and then quickly move on to programming in C/C++. An Arduino Mega is 24 pounds these days on ebay (http://cgi.ebay.co.uk/Arduino-MEGA-ATMEGA1280-Broad-USB-cable-/290430243459?cmd=ViewItem&pt=LH_DefaultDomain_3&hash=item439efdd283#ht_688wt_1167). If form factor is an issue, get a pro or teensy.