Why, they're both only brokers ( Carole Nash sold out a while ago ) - the chances are that both quotes are from the same insurance company - I would ask who are the underwriters - that is more important as they will be the ones that you deal with in event of a claim ...
You still need to deal with the brokers - even for a claim. I know which one I'd rather have to deal with on the phone.
When I wrote off a bike, I dealt solely with Carole Nash. Whilst that was about 10 years ago, their service was superb and I got the cheque for more than I expected within a month of the accident. I was working in Denmark / Sweden at the time and was very grateful that I didn't have to waste time and effort calling back to the UK.
I'll be loyal to them unless their prices rise significantly (they've always been competitive for me) or they do something to really piss me off.