The Zumo is clearly better than the 2610, in everything - it is just a matter of whether you can justify the extra expense.
I've got a Quest 1 and a 2610 (the latter bought off ebay a year ago for £250) and while I love gadgets................. have not been able to justify to myself, the purchase of a Zumo. To me, the 2610 has a fair bit of life left in it.
Sure, I'd like the tracking facility and the better graphics, but since I'd not get much for the 2610 - it would effectively cost the full retail price of the Zumo to change.
To the people that have shelled out on the Zumo already......... Good for you, you've got a great piece of cutting edge kit
To those that have just shelled out on a discounted 2610......... good for you, you've got a bloody bargain there
Simple as that
Al