The 3.5hp units are nearly 20lbs lighter than the 6hp ones. Part of that is because they only have forward/neutral I suppose. But even at 60lbs for a 6hp that isn't bad at all.
It really is cost... I can get the 3.5hp for $300 less than the 6hp. That said, if the $300 is going to get me a big advantage then I'd go that way.
Even if, in strong winds, I'm barely moving forward that is better than me pulling on the oars and getting pushed back up river.