Buit you should mind the extra time. Two hours each way means that you either start and depart very early and late or that your day in Dresden will be shorter.
Also I'm not sure how Dresden became a day trip destination from Berlin. It's one of the most important tourist cities/towns in Germany and like Hamburg, Munich or Berlin one day visits make little sense (unless you have zero interest in any of the museums). It's a seperate area with seperate day trip options. If you'll never return to Germany, then go for it but otherwise ... I mean Hamburg is even a tiny bit closer by fast train.
Potsdam to a certain degree suffers from the same problem. It absolutely is a day trip from Berlin but the sheer number of palaces and parks means that you can't see it on a day. However usually people find out that they don't have enough time for Berlin and Potsdam ends up with one day which includes Sanssouci palace and park and then either Neues Palais or for no good reason Cecilienhof. The point about the Berlin/Potsdam and surroundings palace ensemble is however the sheer number and variety of them. But since your visit is in winter one day will do since many of the smaller places are closed anyway.
I suggest to make the decision based on your interests. Look what both locations have to offer and go for that one. If you share your inerests the forum can help you better.
Τhank you all for your reply!
Lubitsch you were very helpful. After some consideration we decided to go to Potsdam. The fact that it will be winter it means that it will get dark really soon so as you said we if we chose Dresden we should be there very early and still we wouldnt have much time to explore the city. I thnk we can manage our time better in Potsdam
Keep in mind that you will visit in winter - the parks and gardens in Potsdam will be less charming than in summer, many of the palaces will be closed. Of course there is still enough to see and do there (old town, Dutch quarter etc.) but you need to be sure this is what you want to see and do.
7 days isn't all that long for Berlin and yes Potsdam is the usual destination for a day-trip. There is lots to see in Potsdam (far too much for one day as already mentioned) but obviously It does depend on your interests. If modern(ish)art is one there are two museums in Potsdam, near the station, the Barberini Museum and the Potsdam Museum (with a special exhibition on the November Gruppe this winter). Those two plus the Old Town with Dutch Quarter would fill most of a day, without even looking at the parks.
We have visited Sanssouci Park a couple of times in late winter and the statues are all covered with wooden protection from the frost , but of course the parks can be atmospheric even so. The Neues Palais and Sanssouci will be open most days (NB in any case that Sanssouci is closed on Mondays, Neues Palais on Tuesdays)
eagle2002 and Lynda thank you for you replies!
I 've read many articles proposing Potsdam as a day trip from Berlin and many of them are referring to Sanssousi as the German Versailles. I really liked Versailles and i thought it would be a good choise. I know some of the places wil be closed but stil.
Of course if you have any other suggestions i wil be happy to hear them.
The park is "English-style, meaning mainly trees and grass, there is a small formal garden with terracing below Sanssouci