Zell am See and Kaprun are two separate villages 6km apart sharing one region and one card, so the choice is really lake or mountain.
Zell am See town centre
The medieval core and the Esplanade along the water, with the Schmittenhöhe cable car above and the railway station in the middle of it all. Everything is walkable and priced accordingly, and it is the noisiest base in high season.
Schüttdorf
The southern suburb at the foot of the Areit gondola, a couple of kilometres from the centre and on the bus route. Bigger, plainer apartment blocks, lower prices, and the Seespitz lido — where the Ironman swim starts — at the bottom of the road.
Thumersbach
The quiet east shore, facing the town across the water, so the evening light on the Schmitten arrives on your side. Ten minutes by bus round the lake; comfortable with a car, slower without one.
Kaprun — practical tip
A separate village 6km south, at the foot of the Kitzsteinhorn glacier and the Maiskogel gondola, with the Tauern Spa on its edge. Better if you came to ski or go up the mountain, worse if you want the lake, the shops and the trains.