I believe there are some additional differences...for example the turn signal/blinker lights on the 1710 do not have the metal guard, the 1720 does. You'd need to talk with a dealer or at least go look at them yourself. There was almost a negligible different in price between the GC2400 and GC2600, but when they switched over to the 1700 series, they took some cost of the the lower cost model to make more of a difference between the two.
Have not seen one in person, but online it looks like the 1720 has a much nicer seat as well with arm rests in some pics..... My dealer says it is about 4K difference ( Canadian ) Can't see it being that much of a spread..... ?!