Letchworth is a beautiful park; I can speak from experience and highly recommend it. If you like hiking, it has many miles of trails of varying lengths and difficulty. The campsites are electric only, and there is a nice triple dump and water fill station near the campground entrance. Do be aware that the campground is at the opposite end of the park as the main "attractions" of the waterfalls and footbridge and so forth, a distance of several miles. (This is primarily a consideration if you have a motorhome and no toad; it means you'd likely be breaking camp and driving it to and fro. That's certainly doable, and parking is generally sufficient and reasonably accessible along the way, but it can be inconvenient to set up and tear down daily.)
Watkins Glen is also beautiful, though I haven't been there myself.
Reservations may be a good idea, particularly on weekends and particularly getting towards the end of September as foliage season is approaching or starting. It may help to gauge the availability for the date ranges your thinking of and see if things are getting pretty booked up or not.