If you haven't already seen NYC, it's a must. Good job it's on both itineraries!
I drove up to DC from Norfolk, VA last year. A day, two maximum, is all you need in DC. Once you've seen all the tourist attractions there's little else to do. It was a very interesting day and it was great to see all the monuments, the White House, the reflecting pool, big Abe and everything else you've probably seen on TV.
I went to Toronto in December 08. Brrr, -20 every day! Maybe it was a bad time to go to Toronto but it was dead. There was very little happening and apart from the CN tower, not a great deal to see. I quite liked the chilled alternative coffee drinking jazz scene and didn't do a whole lot else apart from drink and listen to music.
Niagra was an Americanised Blackpool. Hookers, casinos and lots of "24HR BAIL BONDS". The waterfall was cool but it got boring after 15 minutes

There wasn't much to see on the drive from Toronto either.
Of your two options, I would personally do DC, NY and Toronto all in one go and I'd probably cut out the driving as well. There's nothing in between them to make a drive worthwhile. Compared to your recent trip to the west coast, I think you'll find it very underwhelming. Stick to the cities for something to do.
I'm assuming your other half has a big say in this and I don't expect this will sound too attractive but I'll throw it out there anyway. Have you thought of going across the southern states? From Florida, follow the Gulf coast right round to New Orleans. I did that (completely unplanned after getting stuck in an airport) and whilst there was nothing overwhelming to see along the way, the sheer "southerness" made it quite special. There's plenty of awesome beaches on the way if that sort of thing floats your boat. Your lady will probably love New Orleans too.