9N is not too bad, some ups and downs but not scary. Its kind of 6 of one /half dozen of the other for taking 9N or 86 east, probably less then 10 miles difference. Don't know if you're planning to spend time in Vermont along the Rt. 4 corridor, it is nice. But if I were heading to New Hampshire from Lake Placid and wanted a direct route, I'd take the ferry across Lake Champlain @ Essex NY and go north for a bit to I89 south. Just depends on if you want to cruise and relax or get there a bit quicker on main roads. Either way is nice, just be safe and have fun.
BTW, you probably know but the Ironman Triathalon is going on today in Lake Placid and its a zoo there with lots of road closures.