I've never used a Donkey, but I love my FirstWheels City Twin.
My twins are now 2years 3months and still have lots of room in the pushchair seats. I used the carrycot attachments from birth to about 5 months, and have had them parent facing in the seats ever since. We sometimes use it forward facing too, if we're somewhere like the zoo, where the interesting stuff is ahead of us!
The frame is well made and folds easily, though it is big when folded. It is also quite a wide pushchair, though there are only a few places I've tried to go and not fitted (sadly my doctors surgery is one of them!).
The straps seem about the same length to me as my other pushchairs (Loola singles, Britax B-Dual tandem), and they still do up around my 2 year olds with coats and liner/footmuff thing. I have asked for extenders for the Loolas and B-Dual, which are great, so I will be ringing FirstWheels next.
I think the FirstWheels looks great too - when I see another one out and about (quite rare!) or see my DH pushing ours, I am always impressed with its looks.
I think you can get attachments to use a car seat with the frame now, but I've never done that. Not sure which infant carrier it is compatible with.
Good luck with your decision! If you have the budget for a new Donkey, I think I'd get a new FirstWheels and a second hand B-Dual for the days where space is tight and/or you think your toddler will spend most of it walking.