I'd keep it! Not sure what you are paying insurance wise, but keeping mine alongside the S3 is costing me peanuts, partially helped I guess by my age and the fact that it's on a limited mileage policy.
Circa £40 a month to keep it taxed and insured.
It would be around £62 a month for tax and insurance. Which just doesn't seem worth it. I can't really see myself ever walking out and picking the MX5 over the Golf for 95% of trips. If I'm going far, then it's motorway driving and the Golf is more comfortable. If I was just nipping to the shops, the Golf is more practical (and better on fuel). And, for the odd time that it'll be a cracking day this summer then I'd probably rather spend it in the garden with a beer.
I've enjoyed it, but definitely time for it to go.