Legacy 8 bit 8051 family has been de facto standard microcontroller for a long time. But ARM will be the new 8051 in the near future with more performance. ARM is the C51 of the 32 Bit world because ARM microcontroller are widely used in the market, available from many vendors, low price and guaranteed long-term availability due to many suppliers.
The 8 bit microcontroller is no longer suitable for the complex embedded systems with increasingly demand on CPU performance. With 8 bit microcontroller, the developer is virtually confined to use Assembly or C which are not object oriented languages. (Although some vendors do support C++ for 8051, see http://www.iar.com/, they are not suitable with 8 bit microcontroller that has only a few resources. Object Oriented Languages are resource hungry).
Object oriented approaches and the usage of design tools, for example UML (Unified Modelling Language) based tools, allows to shorten the development-time. They also increases the demand for memory and performance.
More detail comparison can be seen on,
http://yan9aye.googlepages.com/C51_ARM_Comparison.pdf
http://yan9aye.googlepages.com/C51_ARM_Introduction.pdf
If you are interested to read introduction for NXP(Philips) LPC ARM7 microcontroller using Keil uVision (pronounced: micro-vision ) and using popular free GNU Compiler Collection (pronounced ga-nu) see one of the following links to download the book ( about 10 MB in size)
http://www.hitex.co.uk/arm/lpc2000book/index.html
http://www.cecs.csulb.edu/~brewer/347/lpc-ARM-book_srn.pdf