Outside is faster, esp. going north from what I know of it, but can be very rolly, and would customarily require a crew of three or (better) four to sail effectively 24/7.
Cruising at 7 knots doesn't enter into it so much as your ability to sail effectively and constantly when wind
vary. Spring can give you NW gales hitting the Gulf Stream
at a sharp angle: confused, lumpy seas can result. Go in closer and you have traffic and shoals to deal with.
I would suggest you take the ICW up (parts of which are wide open and other parts
which sound as dull as the Murray Canal), and consider going "outside" for a portion
of the trip, given a favourable weather window and sufficient hands to work the boat for a dedicated 24-48 hour leg.
As for NYC-Oswego, the current condition of the Erie Canal is unknown to me, but may not be completely restored by the time you get there. Spring rains could further damage the parts
not destroyed by Hurricane
Irene, but I'm sure there is plenty of information online for you to figure this out.
I would suggest three weeks to a month is realistic for the whole trip, considering you might have to lay up for a day or two some places if the weather's snotty. Make sure your engine
, pumps, rig and sails
are in excellent shape and that you have an EPIRB
and tethers and jacklines