>> What is your, and Rockoyster, opinion of Chur - Sargans - Uznach - Luzern then to Interlaken or is it better to go to Thalwil.
Well, that's always a personal choice ;-)
Here are some things to consider in making a decision:
1. Rockoyster's suggestion ;-)
2. The connection via Uznach only comes up if your timetable search is Chur to Luzern. It doesn't come up if you search Chur to Mürren.
I don't know your exact date and time of travel, but my guess is that the reason this connection doesn't come up for a trip from Chur to Mürren is that there would be a relatively lengthy wait at Luzern for a connecting train to Interlaken Ost: probably around 45 minutes. Trains leave every hour, at 5 minutes past the hour.
If you are happy with this, and you are not spending any other time in Luzern, you could possibly use that time to duck out of the station and do a circuit over the Chapel Bridge, along the river, and back over one of the other bridges to return to the station. Or you could take an extra hour, stash your luggage in a locker, and explore a bit of the old city. It is all close to the station.
3. The route via Thalwil or Zürich goes along the Walensee (Lake Walen) and the Zürichsee (Lake Zürich), and a small part of the Zugersee (Lake Zug) which are all scenic stretches. From Thalwil to Zürich is mostly inside a tunnel.
4. The route via Uznach also goes along the Walensee, a short section of the Zürichsee, and also crosses the Zürichsee. When crossing the Zürichsee, if you look out the left hand side of the train, you will probably see an interesting pedestrian bridge that crosses the lake, which is quite wide, but very shallow at this point. I'm not familiar with the scenery between Pfäffikon and Arth-Goldau, but after Arth-Goldau, the train travels along part of the Zugersee and then part of the Vierwaldstättersee (Lake Lucerne). The view from that side of the Zugersee is not as impressive as from the other side of the lake (from where you have a good view of the Rigi), but is still reasonably scenic.