Fast prime for indoors. Suggestions please.

Just a thought by a non-expert.

In replacing the 15-85mm with a fast prime, you gain a few stops of light but lose that good four stop stabiliser. In addition you trade off some depth of field, which may not be ideal depending on what type of composition you are aiming for. In wanting a wider (than your 50mm that is) angle lens, are you trying to get detail across the whole frame?

Moreover some of these primes are reported to be quite soft outside the centre when used with larger apertures, most notably the Sigma 30mm 1.4. How this would be seen in practice I don't know.

I will be interested to hear the opinions of others with more experience than I have.