you probably won't go wrong with either assuming you have dealer support for both.
If it were me I'd buy the Suzuki as it is lighter and has lean burn.
The Honda is 350cc and the zuke is 327, in theory the Honda should have more torque but better fuelling may bring the zuke back in line.
Fuel efficiency on a 20hp isn't a concern tbh, both will be cheap as chips to run.
With that said, I'd buy the zuke between those pair.
Edit....the holy grail of sib engines.....http://www.ebay.co.uk/itm/18HP-Tohat...0AAOSwQPlV~yPB