Zermatt or ???

While Zermatt is beautiful , it is slow to get there with transfers and the weather can be iffy. In addition to hotels that are quite costly. If u prefer not to vist Lucerne again , consider the Lauterbrunnen Valley. It is in the Berner Oberland area with waterfalls, and drop dead gorgeous mtns. The gondolas take u where u wish to go then ride or hike down. Book lodging asap, it is busy in summer.
Another option could be beautiful Engelberg. A quiet village a stunning monastery to tour and a gondola up mtn if u wish.
I would say fly straight to Zurich and train out as soon as u arrive as train is at Zurich airport.

Hope this helps alittle.

It might be a good idea to add those nights to Wengen. There are endless hiking opportunities in the Lauterbrunnen Valley and surrounding mountains. We have spent almost three weeks there over the course of four trips and will go back. More than enough to keep you active. Stopping in another location just eats up time that could be devoted to doing something. If you spend two nights in Zermatt and your one full day is overcast, well, it will feel like a lost day.

I agree with Laurel. Stay two more nights in Wengen. It will be way more relaxing and there is so much to do there. Flying to another country for just two nights will feel rushed.

If I could get two nghts in Zermatt, I would go there

I personally love Zermatt but only go if the forecast is for clear weather...otherwise it's really not worth going.
Take the gondola up to the Trockner Steg and then walk past the lake up to the glacier (or the place where the glacier used to be...it's melted and totally gone as of a few years ago) and you have an amazing view of the Matterhorn towering above you. Tons of little Valais blacknose sheep graze up there. If you're feeling adventuresome you can keep going up to the hutte that is on the side of the Matterhorn. Tough hike but a unique place to visit.
I also recommend going up to Glacier Paradise...one of the most amazing gondola rides I've ever taken and crazy views from the top.
Take money...our day in Zermatt (we didn't spend the night) was one of the most expensive days I've ever had in all of my travels. Worth it though!

Trains through Paris are fast - 2.5 hours London-Paris and 6.5 hours Paris-Wengen. But tickets can be expensive. The best rates you see online today for your travel dates are the best you can get - they'll only go up as more seats get booked. If you're planning to get a Swiss Pass or Half-Fare Card, then that would kick in at Basel, the official border station and a common connection point, so you'd only have to buy reserved TGV tickets as far as Basel.

Engleberg is also very nice. You can take the gondola to the top of Mt. Titlis. There are many hiking trails. There is an Abby that is famous for its cheeses and the valley is very beautiful. We stayed at the Ski hotel Engleberg. It was very nice.

Thank you all for the detailed responses. I really appreciate it. Haven't decided what we will do but obviously have to do it quickly and your input is invaluable!

We just came back from Switzerland yesterday and highly recommend a day trip to Aareschlucht (just outside Meiringen) and walk the Aare River Gorge along the catwalk. The area is beautiful and a quick train ride from Interlaken Ost, or you can do the short hike to the area from the Meiringen train station. You won't be disappointed.

http://www.aareschlucht.ch/en/Angebot/125_years_Aareschlucht#.WV4wVaOWzIU