72625 posts
Hi Cotts4, and welcome to MySwissAlps!
The travel time from Mürren to Brienz or Thun is about the same and involves the same number of connections.
If you intended to go back to Zürich on the Golden Pass route via the Brünig Pass to Luzern, Brienz would be more convenient. If going back via Bern, leaving from Thun involves less changes of train than leaving from Brienz.
Both lakes are beautiful. Thun is a much bigger town than Brienz. Thun is closer to the Stockhorn and Mount Niesen, and the Golden Pass route from Spiez to Montreux. Brienz is closer to attractions such as the Brienzer Rothorn, Aare Gorge, Reichenbach Falls and Meiringen (for scenic trips on the PostBus).
However, travel time between Thun and Brienz is only an hour by train, so both are pretty convenient for sightseeing in the area, and transport is excellent.
If it were me, I would make my choice based on what accommodation was available that best suited my needs in terms of price and amenities and was located within an easy walk of a railway station – whether that be somewhere on Lake Brienz or somewhere on Lake Thun. When I myself was looking for accommodation in that area, I was unable to find anything suitable in Brienz or Thun, and ended up staying in Meiringen instead, which worked out well for me.
Alpenrose