The risk not starting with assembler is that so many intricacies of the hardware will be hidden from you and leave you hung because "according to K&R" your "C" code is correct. If you were to program PCs, I would say go with "C", but to learn the '51 nothing beats assembler.
PS whatever language you start off with you will need the 80C51_FAM documents at http://www-us.semiconductors.philips.com/handbook/...us_66.html

welcome, have fun

Erik