Hi Lily, and welcome !
I live in Gozo, and visit Marsalforn as little as possible !
(I think Marsalforn is not as bad as the worst parts of St Pauls Bay/Pacevil(l)e, but fail to see any redeeming features aside from the beach and the Calipso Hotel. Xlendi is less developed, largely through a fate of nature, meaning one side of the bay is too steep to build cheaply on. Other parts of Gozo are far better, but you will probably get others disagreeing with all of the above ! I think Gozo is one of the most wonderful places in the world, but Marsalform plays no part in that)
beach - Marsalforn
restaurants - Xlendi
relatively unspoilt and quiet - Xlendi
which hotels/apartments are you deciding between ?
also think about staying at The Grand Hotel, Mgarr, near the harbour. September is a great time to come here - the sea is warm, hopefully there are no jellyfish, and its not packed like in August
ps
best beaches are Ramla and San Blas
best restaurants are Sammys at Mgarr, Noel and Sandra's at Mgarr Ix Xini, It Munn in Victoria, Jeffries in Gharb and D-Bar in Qala
best bars are Gleneagles in Mgarr,Cafe Jubilee in Victoria and Rosina's in Sannat
note Xlendi and Marsalforn don't feature in either list !