ARM is the most widely used embedded architecture and covers an enormous range of devices from multiple vendors and a wide range of costs. That said there are significant differences between ARM7, 9, 11, and Cortex devices - especially Cortex. However if getting into embedded systems professionally is your aim, ARM experience will serve you well.
8 bit architectures are generally easier to use, but often very limited in both memory capacity and core speeds. Also because they are simple to use, 8-bit skills are relatively easy to acquire, so it is a less attractive skill for a potential employer because it is easy to fulfil internally or with less experienced (and therefore less expensive) staff.
However if this is a hobby rather than a career, the low cost of parts, boards, and tools, and ease of use may make 8 bit attractive. I would suggest AVR simply because it is supported by the free avr-gcc toolchain. Some 8 bit targets are supported by SDCC, another open source C compiler. I believe Zilog make their Z8 compiler available for free, but you may need to pay for the debug hardware (although this is relatively inexpensive). Many commercial tool vendors provide code-size-limited versions of their tools for evaluation and non-commercial use, but beware most debuggers require specialist hardware which may be expensive, although in some cases you can build it yourself if you only need basic functionality and low speeds.
Whatever you do do take a look at www.embedded.com. If you choose ARM, I have used WinARM successfully on commercial projects, although it is not built-for-comfort! A good list of ARM resources is available here. For AVR definitely check out www.avrfreaks.net
I would only recommend Microchip PIC parts (at least the low-end ones) for highly cost sensitive projects where the peripheral mix is a good fit to the application; not for learning embedded systems. PIC is more of a branding than an architecture, the various ranges PIC12, 16, 18, 24, and PIC32 are very different from each other, so learning on one does not necessarily stand you in good stead for using another - often you even need to purchase new tools! That said, the dsPIC which is based on the PIC24 architecture may be a good choice if you wanted to get some simple DSP experience at the same time.
In all cases check out compiler availability (especially if C++ support is a requirement) and cost, and debugger hardware requirements, since often these will be the most expensive parts of your dev-kit, the boards and parts are often the least expensive part.