Interlaken is not in the mountains.
It depends on what you want to do, but I prefer to be in the mountains when I am visiting the mountains.
If the only thing you will want to do is ride up to the Jungfraujoch then Wengen could be good.
I prefer to stay across the valley and a bunch higher at Muerren. From Muerren you can look down on Wengen across the valley, and up at all three of the big mountains, the Monch, the Eiger and the Jungfrau. Getting between sides of the valley is easy by gondola and train via Lauterbrunnen.
Interlaken is good if you'd rather ride the lake steamers, have quick access to get away by train and eat at a McDonald's.
When you are searching for a room, spell Wengen (with the "n") and you will have more success.
There are frequent fast and comfortable trains from Basel SBB station to Interlaken. Get off at Interlaken West for boats on the Thunersee. Get off at Interlaken Ost - the last stop - for connections up into the mountains, the McDonald's, and boats on the Brienzersee.
Some trains from Basel are direct; some you need to change trains at Bern.