As someone who finds it difficult to make his own mind up I support the option which allows most room for manoeuvre, i.e keep both. The price of the micra can't have much further to fall and it is, as you say, a backup.

You never know with cars so my feeling would be to make a cautious choice.

As others say keep both.

If you SORN the Micra have you got somewhere to keep it where it's right off the road and not in the way? Will it still be insured (at least for fire theft) and have you thought through how to stop it deteriorating while stored.

Also if it's on 'warm standby' to cover for the Focus but SORN and not insured for road use there's a bit o paperwork to do to get it back on the road. Although you say you don't need it in your new job are you sure you won't change your mind and/or discover that being carless while the Focus is 'on duty' for the commute is a PITA.

If you can afford the insurance/VED for the Micra then I'd keep both and use them interchangeably for the longer commute and general social/domestic use.

My worry with having the Micra SORN is that it might get damaged not being used (I don't know enough about cars: is letting the engine run a few minutes per month while parked enough, along with having the battery unplugged?).

There are other threads on here about what to do if a car is 'laid up'. If you are talking about several months, you will need to think about tyres losing pressure and flat-spotting, for instance. As it's not under cover you must remember to remove all kinds of fallout from the exterior too, especially if it causes paint damage - and the interior could do with occasional ventilation.