Below is a list. I have compiled it in December 2008. Obviously, it's not a complete list, and it's merely a single opinion.
Pros and Cons of ARMs (compared to PIC18)
Pro:
• Ample resources (ROM, RAM)
• Faster program execution.
• More sophisticated interrupt architecture
• Exceptions
• C++ compilers, which might speed-up development and facilitate code reuse
• JTAG
• Many silicon vendors
• Many flavors
Con:
• Smaller mind share
• Lack of small device (the smallest one is 48-QFP)
• Larger amount of necessary external hardware