I don't think there is any difference performance wise. They will fit in place of any SU or Stromberg carb on pretty much any B18 or B20 manifold.

I think the HIF6's are a little harder to tune - the adjuster nut on an HS6 is on the outside and readily adjusted by hand with the engine running, I don't think it is nearly so simple on an HIF6 with it's internal jet. They have a bimetallic arm that supposedly helps richen the mixture when cold. And another tidbit is that is a float bowl valve sticks open an HS6 will pour fuel down onto the exhaust manifold where as an HIF6 will dump the overflow into the intake manifold.