Long versus short domains

This is potentially a little off topic but I'm hoping someone might have some wisdom!

I'm starting a new site for a client called the South Central Gospel Partnership. We've got southcentralgospelpartnership.org and .org.uk registered and also scgp.org.uk

I figured scgp.org.uk would be better for email addresses and for displaying on printed publicity as it's shorter but I wonder if the longer address is the better canonical (is that the correct word?!) URL for things like SEO etc. So I'd set scgp.org.uk to link to the longer address.

Does that make sense? Is that a good way to do it or will that cause me issues down the line?

Any thoughts?