Masturbation is generally a solo occupation. Business. Pastime. I'll start again.
Most people discover masturbation on their own, alone, and continue practicing it in solitary. Not to say that people outside prison walls don't... uh, I'll start again.
No one expects the Spanish Inquisition!
Sorry. I'm watching Monty Python on DVD and had a brain leak.
Seriously. Masturbation is a private thing. I was surprised and pleased when someone else grabbed me for the first time. It all seemed very intimate and special, freeing. Being able to share myself like that opened me to many possibilities.
On the other hand, masturbating myself in front of someone for the first time was pretty good, too. Again it was the sharing that did it for me. "Doing" someone else was great as well.
Asking which is preferred is like asking which kind of fruit do I prefer; apples or oranges? A handjob means I have somebody to talk to afterwards, so if I'm stuck on The Island of Either-Or, I vote handjob.
aaron