I've been wondering the same thing for when our next bubs comes along. A few things to remember, do you go for long walks etc? the board etc are not really good for that. Also you mentioned "walking by your side in 6months".....my DS is 2.5 & still does not always do this, therefore I'd rather he be restrained to say my leaving the pram & running off after him. Also if you go on holidays etc & they get tired you dont really want to have to carry them.
The other thing for me is weight, most double strollers only go up to 16kg or so each side. My DS is already approx 13kg or so. Some double prams are the same. Same goes for the toddler seat, my DS moves about a bit also & I don't like the idea of him making the pram top heavy.
From my research & what I've been told the Phil & Ted's side by side are great...esp if you like to use it for longer walks. I've already got an exp pram so am looking on ebay for that or a Mountain Buggy side by side.