Dresden to Nuremberg by train...which option ?

I would make the longer stop at Erfurt, assuming that you are not cutting short the other destinations. Rick's guidebook describes a self-guided walk of the old town that takes about an hour, plus a little more time from the train station. Visiting the monastery is a one-hour guided visit, or going up the hill to explore the citadel would take some time, as well.

Does Erfurt (or Leipzig) deserve a stop ? For how long ?

Erfurt is one of the few big German cities which wasn't destroyed in WW2. To me the city is much more attractive than Nuremberg for example, which was leveled in WW2. Erfurt offers a very pleasant old town with a famous bridge, one of Europes oldest synagogues, a impressive cathedral hill, and a hearty and good cuisine. If you want "Old Europe" you need to see it.
Leipzig is often called the "New Berlin", because it's young and trendy. The old town is better preserved than Munichs or Nurembergs, but it was always a trading town and basically the whole old town was rebuilt in the late 19th century to accomodate the trade fairs. There are no real highlights apart from Bachs church, the church were the revolution of 1989 started, the huge train station, Germanys most famous restaurant and the zoo.

An ICE is definitely more comfortable/enjoyable than most regional trains. However, the newer, double-deck regional express trains are pretty comfortable.

Option 1 is the fastest and cheapest. I took trains from Dresden to Freising (Munich) via Hof in 2012. Nothing wrong with the route, but nothing spectacular, either. If I recall, Dresden to Hof was on a newer, comfortable RE. Hof to Freising was on an ALX train with side aisle and compartments, which I like, but your trains could be different.

Option 2 costs the most and takes a lot longer, but you get to (have to) spend 4 hours in Erfurt. Could be very good or very bad, depending on if you consider four hours in Erfurt a plus.

Option 3 takes a little more than 1 and costs more, but is by ICE.

When I made the trip in 2012, I was staying in Bad Schandau, less than half an hour east of Dresden. My ticket (single) included the leg into Dresden on the EC coming from Prague, so I qualified for a Saving Fare ticket for only 29€. Otherwise it would have been (at today's prices) 72€ with a standard regional ticket or 44€ by QdLT. I don't remember having any option but the standard ticket, so maybe they didn't have QdLT then.

1) Is ICE more comfortable/enjoyable than regional train ?

Yes it is.

2) Does Erfurt (or Leipzig) deserve a stop ? For how long ?

Yes, they both deserve a stop longer than what you have avialable, see my answer to question 3 below.

3) what would be your choice ?

If I had to choose and I REALLY wanted to check off one more city I'd pick option 2 with a four hour layover in Erfurt. 22 minutes in Leipzig or 2 hours in Erfurt is not really enough to do much of anything. With Option two you can at least arrive in Erfurt late morning, grab an unhurried lunch and spend an hour or two walking around the old town. It's a 15min walk from the train station to the city center (just follow the street car tracks from the station).

DJ

If you go the QdL route, you could stop and visit Bayreuth. Bayreuth is home to the Wagner Festival which will start about that weekend. Beautiful old town pedestrian shopping district, the Margravian Opera House (not the festival site which is atop the Green Hill opposite direction from the train station) and the Margravian Palace. Also the Wagner museum in his original home of Wahnfried.

There is a beautiful limestone gorge the train passes through between Bayreuth and Nuremberg, called the Hersbrucker Alb, along the Pegnitz River.

Here is a sneaky (but legal) way to use the QdL ticket but get an earlier start. The leg from Dresden to Hof leaves at 8:53, too earlier to use a QdL ticket. But if you buy tickets to the first stop at Tharandt, it leaves there at 9:07, so then the QdL ticket is legal.

"Here is a sneaky (but legal) way to use the QdL ticket but get an earlier start."

It's not sneaky, the Bahn even suggests it.

we bought our tickets last night, I think we got a good deal.

Train (IC) leaves Dresden at 9:20.

Arrival in Leipzig at 10:28. We have a transfer there and only 7 minutes to do it. We arrive at track 13 and leave at track 13, I guess they are not too far ! Can you reassure me about that ?

Train (ICE) leave Leipzig at 10:35. Arrival in Erfurt at 11:18. Then we have 3 hours before our nex train. Time to take a walk and have a quick lunch. I guess we can found lockers at the train station for our luggage, I'm I wrong ?

Train (Ice) leave Erfurt at 14:34 and arrive in Nuremberg at 17:28.

I spent a couple of nights in Erfurt last year, it is a good city for the reasons Martin mentioned. Definitely have a look at the synagogue. Martin Luther is important in Erfurt's history as well.

At Leipzig you arrive on track 13 and depart track 12 so it will be straight across the platform.
Last year, I was 10 minutes late arriving at Leipzig from Berlin and they held the train to Erfurt until everybody made the transfer.

Lockers at Erfurt are next to the escalator going to track 2 on the ground level. The German word is "Schliessfach". Icon is a suitcase in a box with a key above it.
http://www.bahnhof.de/file/bahnhof-de/6493602/Vp3aOAgvu6q-9OKkeJm67D3BROs/6503766/data/Erfurt_Hbf_de_PDF.pdf

Here is a plan of the Leipzig Hbf.

I'm assuming you bought an advance purchase, non-exchangeable SparPreis ticket. If for some reason (a fault of the Bahn), the first train is late getting into Leipzig and they don't hold the connecting train, the Bahn will put you on later trains gratis. There is no penalty if the missed connection is the fault of the Bahn. I know, it's happened to me.