In addition to other answers that you have already received (price versus
stability and shock resistance)

A resonator is usually smaller (unless you buy a very expensive crystal) 
A resonator can have the capacitors included in a small physical space
thereby using much less room than a crystal. With a surface mount PIC the
room saving can be substantial. This only matters in tightly space
constrained designs. 

----------
From: Ricardo Ponte G <rniniver@CANTV.NET>
To: PICLIST@MITVMA.MIT.EDU
Subject: What«s best: a Resonator or Crystal ?
Date: Saturday, 25 July 1998 16:06

        I was thinking about it:

        What«s the advantage of a Resonator to be use as a Pic«s signal
clock?

        and what about a Crystal ?

        What do you think ?.
----------