Yellow - I know a 'mega multiple momma' with 6 children. She does slings and carriers. She decided with DC4 to get rid of all her pushchairs and just carry. But by DC5 two years later (and subsequently DC6) she did go back to pushchairs.
I'm not a fan of carriers and slings, so cannot give any opinion myself.
Have you used a buggy board before? Mine lasted about 4 trips before I sold it for being rubbish to use.
It gets in the way when pushing a pushchair meaning you have to over-extend your arms and lean forward to push. OK every now and again but not every time. Maybe I just have short arms, but was not a sustainable position for me to use when out and about day in and day out.
Also makes a hassle out of folding up the buggy, since you have to disconnect the board each time. Not a problem if you leave yours up, but my pushchair goes in and out of the car boot daily.
Finally - my children didn't like it, so served no purpose anyway!
I've always turfed mine out of the buggy at 2 1/2. I would go for a double buggy until then. My best friend had a Phil & Teds and I loved her double. I have the Graco Stadium, which I wouldn't recommend 'cos it's so bulky. Cheap though, and fit Group 0 car seat.