Run what he or she runs. If you would be most likely to ask on a mailing list for Linux help, read the list for a while and see what the most helpful posters run. Any technology distinguishable from magic is insufficiently advanced. Most people would say this, although the stability of Debian is in the eye of the beholder. Oops, I didn't mean to put Mandrake so far ahead of most of the other distros.
Many people find Mandrake easier to install than most distributions.
The Duke of URL has been writing reviews of new distributions as they appear. Read a few reviews, and that's the same as a comparision. Links to the Duke's reviews and others are in the past several issues of LG in the News Bytes column. Linux Journal has a table comparing several distributions. It was written July 2000.