Right, here goes. Background - I bought the MB first when pregnant then decided the E3 sounded like the ideal buggy so bought that just before DD arrived. There's 18 months between my two and I had a c-section with the second.
Main problem was that the second seat didn't recline for the baby and she didn't seem comfortable in at 4/5 months - squirmed, screamed, etc. Was a fight to slot her in and when I banged her head once trying to get her in that was the final straw.
Also didn't like the way main seat dipped at bum so that DS's knees were higher than his bottom - bad posture.
Didn't feel comfortable to push up and down our hilly streets and over uneven pavements. Felt like the centre of gravity was out and I had aching shoulders. Hard work to lug it in and out of car especially after lugging child and baby in and out of car.
Didn't like the fact that people couldn't see DD. Not only did I want to show her off but I wanted her near me and once when DS was in supermarket trolley with DH and I was pushing DD in 'empty' buggy, a couple of people crashed their trolleys into her.
Good points - the removable carrycot was great, especially for things like music groups where DD could stay asleep and be put out of reach. Good too for a winter baby. But it was small. It meant she was out of reach of occasional outbursts of jealousy from DS too!
MB - We bought a carrycot for the MB so at weekends (when DH could put the buggy in the boot) we used that. It was a bigger carrycot so DD was flatter for longer. After I recovered from c-section we found ourselves using the MB more than the E3. Both children look more comfortable in MB. Easier to collapse too. Need a particular car for it though. Fits in Peugeot 306 estate with ease but not so easily in Audi A6 estate.
Have a single Nipper and alternate between using the two most days. Plan to get a buggy board for the Nipper for our holiday.
Phew, hope that helps!