In SX Microcontrollers, SX/B Compiler and SX-Key Tool, George Herzog wrote:

Parallax is just trying to keep its parts inventory simple - a fast resonator and slow resonator.

You can run SXes on just about any crystal or oscillator up to 80Mhz, maybe 100Mhz too.
You might even use a 555 to run it or a flip-flop circuit.  You can borrow timing from another device too.

Resonators are just the easiest, smalllest device to use.  0.5% tolerance is available while the slow internal oscilator varies up to 6-7% due to temperature changes [no good for a UART].

Crystals can provide even tigher control, but gosh.....
Crystals come in series cut and parallel cut.  Crystals come in a lot more frequencies too.
You might like a crystal to deal with a particular oddball frequency [like video timing]

But it gets complicated, while series cut appear to not require additional capacitors, the parallel ones do require two caps.
Then the problem is how to find out which value those capacitors should be.  

And, how do you know your crystal is series or parallel cut?  From what I have read, generally, series cut are limited to about 20Mhz.  Over that it is likely a parallel cut, but below that - well, I don't know.

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