I think from what I have heard you will struggle with a city select with a nearly 3 yr old, it becomes heavy to handle as you add more weight.
My two pence worth (esp as tiggy and I have different priorities sometimes so we come out with different answers) -
B Dual - I liked the b dual, didn't really find the lack of footrest for the toddler a problem (the Twone is also like this if you do happen to look at it) but I did find the whole thing too wide for my life. Otherwsie it was nice to push, had basket space etc. The lower seat is a little too reclined but it can easily be fixed.
iCandy Peach Blossom - I had the blossom 1 with the smaller non upright lower seat, I'm trying to figure out how you'd do the single to double swap with a peach because the lower carrycot isn't recommended for use as a single, indeed I don't even know if it physically fits on the blossom 2. It didn't on the blossom one as the adaptor spacing was different on the lower seat. It's a nice pushchair I just can't see it working in your situation, if you were leaving it as a double all the time then it would be different.
Mountain Buggy +one - same as tiggy basically, you would have to be very clear about who was going to sit where at any given stage. I think it's probably less than ideal for you in the sense that when using as a single you are still going to be pushing round an empty seat at the front, as you would a p and t, the only difference being the baby would be behind it as oppose to underneath.
Jane Twone - I have a Twone and love it but I have also cheated and there are compromises. It's lovely and light to push and the main seat is excellent. In your situation you would have the baby in lower position in carrycot, toddler in upper seat, then leave upper seat at nursery, drop carrycot onto upper position - very easy and no extra bars or anything. When you got to the seat/seat set up you hit the age old tandem problem - baby in bottom seat so can recline any time to sleep, pushing round buggy with top seat empty when toddler is walking. But if you put the baby in the top so the toddler can be in and out of lower seat more often than not the baby can't be reclined for sleeping when lower seat is occupied. This is a PITA. Also for my twone I obtained a second main seat until, which leaves a much larger gap between the two seats, with the smaller lower seat on they are very close to the point you have to use it one notch reclined really. My toddler can climb in to the lower seat but not out (not entirely a bad thing in my book!) I have a Duet in at the moment, with the air tyres I expected it to be easier to push and take cobbles and bumps better however it does not so I can sort of recommend the Twone for places with uneven surfaces... In as much as I can ever recommend it given the compromises 
iCandy Pear - no experience of so cannot really comment. Reportedly heavy to steer with weight on board and big turning circle.
Micralite Twofold - looks great, still haven't had sight of one, lots of options, seems like it will work for all stages but can't be sure until it arrives and some reviews start to come through.
P and T promenade - have only seen and played with one of these on shop floor. It felt long because of the way the handle extends, I'm not sure what this means for things like tipping it up kerbs with children in etc. I like the theory of the convertible seats etc. I did think it seemed very expensive compared to a b dual, which when I viewed them side by side I would most liken it to.
Babystyle Oyster Max - Personally I would get this in a heartbeat, it seems to do all you require and is easy to push and use. Again no storage but you can get around that with add ons. 