The 5% return. The negative returns throw you further from the geometric mean [nth root of the product of(1+rn)] your geometric return is only 4.5% for the second set of returns. That and the application of correllation to returns is the basis of modern portfolio theory.
The second reason is that if something bad happens to the company you lose your job and your nest egg if it is entirely in company stock.
So, ASAP you should go log into your 401k administrator's [plan sponsors] page (or call if they are out of date) and find out what you are invested in, and speak with a professional (or do some research yourself) on how to allocate your assets to assure you a more prosperous retirement.
That is all, enjoy your weekend, but please go look at your 401k on monday.