We were at the Wyndham Old Town Alexandria two weeks ago, and we too were wondering which one to stay at next spring. I couldn't get much info from the staff about National Harbour, and I didn't want to do an 'owner's survey', so we drove over by ourselves to have a look at the new resort. From the outside, construction looks almost complete, and we could see them installing curtains in what I think will be a demo unit.
My first impression of National Harbour was that it seemed a bit isolated, and not within walking distance of anything outside of National Harbour itself. The ambiance is totally different from Alexandria - pristine vs quaint, I guess you could say. There didn't seem to be a lot going on there in Sept. We were the only ones out on the pier at noon on a Friday. I am sure there will be more activity by July. The Wyndham is located a couple of blocks up from the waterfront. The big draw for me is that the new resort will have three bedroom/three bathroom units.
From Alexandria, we took the metro into Washington to visit the museums and monuments during the day, but we ended up spending evenings wandering around Alexandria. This might not appeal to your younger crew, but I loved it. We bought daily Metro passes @ $7.50 ea. If there are five of you heading into Washington, it might be more cost effective to drive and find parking. (I don't know how challenging it is to find parking in Washington though). And then you also wouldn't have to haul your purchases on the Metro...
All that being said, I am still undecided myself.....
...From the Capital of Canada