fly from nice to barcelona or train?

Also I arrive 9/18 and leave on 10/5
Any advice on where to stay in between stops would be great.

Before anybody can give you sensible advice, you need to explain more about your interests and why you have chosen those destinations. As it is, you have 16 nights on the ground, with 4 widely spread apart countries and 6 or 7 possible stops within them. Lots of your time would be spent on trains. With no further information, I would say, "bad plan."

Thank you , that is why I am thinking it would be smart to fly from Nice to Barcelona.
I fly into Frankfurt and I fly out of Barcelona. I am staying with family near Frankfurt than traveling by train to Lauterbrunnen Switerland and staying there. I want to go to the coast of France and so I picked Nice and I want to explore Barcelona.
I have never been to Europe and I am going solo.

Here is the basic structure of travel from Nice to Barcelona, if done directly:

Train takes at least eight hours, with at least one change of train. In mid-September it will cost at least 80 euros.
Flight takes about 75 minutes. Add in a couple of hours at one end to get to the airport and check in, and an hour to deplane, retrieve luggage and travel to the centre of Barcelona. But you can do it for around 40 euros in mid-September, plus local transport tickets and whatever in-plane services you buy.
Prices are lowest when bought far in advance; a purchase close to departure can be double or even more. True for both planes and long-distance trains.
Main-line train purchases include a seat reservation for a specific day and time. Tickets that can be switched are more expensive.
Resources:

www.seat61.com

https://en.voyages-sncf.com/en/

www.skyscanner.com

Thank You!
Any thoughts on staying a night in Milan or instead a night in Genoa?
Also Bern? skip it or stay a night

If what you are after is "the coast," you might enjoy French Catalonia and the area around Perpignan more than Nice. Nice is pebble beaches and movie star territory.

You don't sound all that committed to Milan, either; I think it complicates your itinerary, so unless you are really drawn to it this trip you might save it for another time. Not judging, just observing.

To answer your question, though, flying would be way more efficient, but there is a boatload of great stuff to see between Nice and Barcelona, so if you are of a mind you could construct a multi-day itinerary of medium-sized hops that might be a lot of fun. It depends on your travel style and what interests you.

I don't personally feel that Bern rates more than a few hours; it is certainly not in the same league as your other destinations. If you skip Milan and Nice you could head south and stop in Burgundy or Lyon; Provence is also great.

I would lessen these one-night stops as much as possible and just get to your four main areas: Frankfurt, Lauterbrunnen, Nice, and Barcelona. Once you are set up with a base in each, you can look to do day trips. You should fly when possible to save time. EasyJet flies Nice to Barcelona. I did it last year--easy.

There are wonderful places to visit everywhere but you need to focus on only a few and make the most of them.

I don't recall all the details now , but four years ago we did 4 weeks in Europe , London - Paris- Nice- Barcelona ( and from there to Tossa de mar small coastal town 1.5 hrs from Barcelona ) to Dublin- London .

We took trian from London to Paris and from Paris to Nice , but we flew from Nice to Barcelona as price aside and time wise it was cheaper and faster .

Thank you for all who replied with great advice! I am very excited and nervous about my trip.

For Nice to Barcelona ...
8hr 20min is your best train time, with a change in Montpellier.
I usually start looking at direct flight alternatives when the train journey exceeds 6 hours.

I'm looking at going from Lauterbrunnen to Nice and my reason for stopping a night in either Milan or Genoa was because the train ride is long. Any recommendations? Flights between are too expensive. Also when you get on the train can you pay to get all the way to your destination or if it stops do I have to get off and get another ticket to go to the next stop? Thank you!

For Lauterbrunnen to Nice ...
8hr 57min is your best train time, with 3 changes along the way.

RE 124
Dep 07:03 LAUTERBRUNNEN (Switzerland)
Arr 07:24 INTERLAKEN OST (Switzerland)
RE 6711
Dep 07:29 INTERLAKEN OST (Switzerland)
Arr 07:50 SPIEZ (Switzerland)
EC 51
Dep 08:05 SPIEZ (Switzerland)
Arr 10:37 MILANO CENTRALE (Italy)
EC 35043
Dep 11:10 MILANO CENTRALE (Italy)
Arr 16:00 NICE VILLE (France)

I have to say the whole "gotta spend a night in Milan because Nice" thing is an argument against Nice. It's going to take a huge bite out of your trip.

Consider these alternatives: Trade Nice for Provence, which is a 6 hour train ride, or Lyon, which is 5. Continue to the beaches of French Catalonia, or pass them by and go straight to Barcelona.

Maybe fly from Zurich to Nice, and schedule a stopover in Provence.

18/9 Arrive Frankfurt/Wetzler (3 nights)
21/9 Train to Lauterbrunnen (3 nights)
24/9 Train to Zurich (1 night)
25/9 Fly to Nice (3 nights)
28/9 Train to Provence (3 nights)
01/10 Train to Barcelona (4 nights)
05/10 Depart Barcelona

Nine hours and 3 changes of train between Lauterbrunnen and Nice doesn't worry me and you'd have to make a couple of train connections to reach any airport, too.

Flying from either Zurich or Geneva compares pretty well on price. Since nonstop from Zurich are only by Swissair, sample fares on Sept. 29 are around $130-145. Flying from Geneva gives you a second airline choice in EasyJet and fares from $40-100, depending on departure time.

Tickets purchased in advance for Italian and French trains are not good on later trains, with very few exceptions. In addition, the direct Milan-Nice train is operated by Thello (from €15 if you book it now), whether you buy it from them or from Trenitalia. Trains via France are likely to be more expensive, even if booked ahead, such as the direct TGV Lyria from Geneva to Nice at 12:40 (from €40 if you book it now). Prices for the Swiss train tickets are similar in either direction.

If you were thinking of traveling by train in order to use a Eurail pass, I'd advise your that there are several issues to using a pass on your route. The Thello train is not covered, so between Milan and Nice you'd have to take an InterCity train with additional connection at Ventimiglia. And the TGV Lyria and France-Spain TGV seat reservation fees are highest around, with limited availability.

Thank you!