Hi,
Sound&Drumland is a chain store, they have the typical middle-of-the-road stuff but nothing special, and they're not very knowledgeable and have bad service, I don't buy there anymore. They are the biggest though.
The other two I can also recommend.
Fair pricing and good service you get at
http://www.borkowsky-berlin.de/And there are
http://www.berlin-guitars.com/ and a vintage & 2nd hand shop named "Checkpoint Music" at Kochstr. 3 (no website).
And then there are some small luthiers that sell some other stuff, especially Deimel I would check out for some booteek effects. But these are small shops that are not always open, you might want to give them a call first:
http://www.deimelguitarworks.de/http://www.nickpageguitars.com/http://www.luk-guitars.de/http://www.saitenreiter.com/Berlin doesn't have THE large cool go-to guitar place to check out, unfortunately, but all these shops should be able to give you some help.
Enjoy your stay!