We had similar decision 2 years ago - were renting in Chalk Farm, and were looking at 2-bed maisonettes there or 3-bed houses in Kensal/Willesden. We bought the latter.
It has been good here - I really appreciate the garden and ground floor access, and it is a lovely friendly area to be in with kids. I don't know WG/Dollis Hill much, but depending on which side you were you would possibly have Roundwood Park, cafe and kids activities on College Road, willesden rec with swimming pool, bouncy castle etc., shops and cafes on Chamberlayne Road
However, if I had to make the decision again I'm not sure what I would do. We rarely use our 3rd bedroom as it is in loft conversion, so would have been fine with 2 for a good while (we have 3.5yo and 21mo who happily share), and a ground floor flat would have given us garden, plus if you are close enough to nice parks and playgrounds and friends' houses, garden doesn't matter so much.
I lost contact with everyone I made friends with through DD in Chalk Farm, and although I have made new friends here, it takes a while to settle in anywhere new. I also miss the centrality and I miss easy access to the Heath and Primrose Hill and a bigger selection of shops and cafes and so on.
It depends what is most important to you, although I know sometimes that is hard to work out until you are missing something!
I would have thought given your kids ages that school would be the most relevant factor at the moment?