I really needed a double pram when E was born - Iz was still too small to go into a toddler seat. But if I was in your position with the age Mason will be, I would definitely be going the single with the toddler seat. These days Iz would much rather walk than be in the pram and never sleeps if we're just cruising around the shops. The only proviso to this is if you go for long walks - I still love my big double for taking the girls for walks (I'm talking 1.5 hour walk) because even if Iz doesn't sleep, she's gets into that trance state and the extra support is brilliant.

You had a HAB with Mason didn't you? That will probably buy you an extra couple of months anyway with your new bub being in that and Mason in the pram.