Hello frenziednester, if its a 3 wheeler you need then go for the Mountain Buggy Urban.
The Quinny brand is manufactured by a company called Dorel. I sell nursery products and Dorel has THE worst reputation for quality (wheeled products, not car seats) and customer service. Loads of their prams end up being sent back for repair over and over, just a waste of your time and money.
That said, I love their car seats. All very well rated in the independent tests and safe.
The Maxi Cosi Cabrio also fits on the Mountain Buggy, so no compromise there.
I assume this is for a new baby, hence the car seat? What do you intend to do with your 18 monther? Mountain Buggy are one of the few products strong enough to take a Kiddy Board for either of your older two when you need it.
In fairness to the Speedi, we do see less problems with it than most other Dorel prams, but for the money spent its just not got the quality.
MB is really easy to fold and unfold so also fits that criteria.
This is all just my opinion. I'm sure somebody will be along soon to tell you of their experience.
Good luck!