I'm a pretty avid skater, but I find that having both is ideal if you're gonna go entirely human powered. Bikes are more versatile with less-than-ideal terrain and inclement weather (but if you're prepared correctly, skating in the rain can be a blast!), and skating has a MUCH steeper learning curve. Also, skating takes approximately twice as much physical effort per mile, and for most people a bike is about 20% faster. That said, my usual skate session is around twenty miles and is so much more fun than anything I ever do on a bike.

There are upsides to doing both, however... mostly the same clothing, helmets, lights, etc., you're already intimately familiar with the pros and cons of all the roads in your area, and you can always throw your skates in your panniers.