Inbetween diaper sizes

I know Pampers and Huggies run slightly different, so know people use one or the other when they're inbetween sizes.  Which way does it go?  DS is outgrowing his size 3 Pampers, but the 4s are still pretty big.  So should I try a 3 or 4 Huggies?  It's been so long since I've had to worry about this I can't remember!