External 32.768 kHz crystal vs. internal RC oscillator

[Eliot Stock]

What are the considerations when choosing between using an external 32.768 kHz oscillator and just using the internal RC oscillator as the low frequency clock source?

Here's the crystal we're using so far. Not cheap.

www.digikey.co.uk/.../en

[Susheel Nuguru]

Have you read this? if not there is a good explanation there. Go down to section Choice of 32 kHz clock source