I've met an equal amount of both in my life, though in my age group (born 1991) Jessica was more common, and I've seen more Gen-Xers named Jennifer.
As far as sound by itself is concerned, I prefer Jennifer. It really is a pretty name, just completely played out.
Please rate my personal name lists:
www.behindthename.com/pnl/69381www.behindthename.com/pnl/69381/109399www.behindthename.com/pnl/69381/91835http://greens-end.myminicity.com