Well, either will get you to where you want to go without too much fuss - once you understand how it all works, and are prepared to get a little frustrated on the way!
I've had a Quest, 2610, and now a 660. I could be biased, and say just go for the 660, but it will come down to what you really want out of it.
I think Arsey said he managed to find a new 550 for £210 - I paid £499 for my 660. So if price performance is paramount to you, the 550 would get my vote. If the latest technology floats your boat, go for the 660.
There are some teething troubles/quirks with the 660, that's true, but it could be argued they're just that, and don't actually affect its prime purpose of getting you from A to Z, via B, C, D... etc. They'll get fixed soon enough.
Apart from that, it's all been said in this section, really.
Good luck!