An external x-tal will "always" be more accurate than the internal oscillator on a PIC.

On the 16F690 the internal 8MHz oscillator is specified as +/-1% under the best conditions (that doesn't mean it can't be better than that though) and the low frequency, 31kHz oscillator is labeled "uncallibrated" and no furhter specification can be found in the datasheet - as far as I can see.

Your typical €0.20 watch crystal is around 20ppm witch is 500 better than the internal 1% oscillator and if you pony up €0.60 you can get a 5ppm one which would then be 2000 times "better".

So the answer is that running TMR1 off of an exernal 32.768kHz Watch crystal is "better" (for timekeeping purposes) than running it off the internal oscillator. But if you're ALREADY running the PIC on a 4MHz crystal you might not gain much by using a separate crystal for the timer - except that the timer will keep on running when you put the device to sleep.

/Henrik.