The RF is defifitely much better lens than the old 135/2. Very sharp and contrasty wide opened, very fast focusing, very reduced aberrations.
But I do not think it is worth its price when there is optically also excellent Sigma 135/1.8 for much lower price. Plus it is fully metal constructed.
I have EF135f2 and Sigma Art 135/f1.8. I bought RF135f1.8 from B&H Photo Nov 6 just a couple of days before the official announcement yet I am still waiting to receive mine after more than 3 months of deliveries. I bought the RF lens because I expect will focus faster and better as well as having IS. My other two 135 lenses do not have IS. I have RF70-200F2.8L IS and it has has IS and speed. However, f2.8 is not as good as f1.8 sometimes in low light and providing bokeh pop.
Yes, the instant availability of Sigma is another plus.
My Sigma 105/1.4 is roughly on par with the AF speed of 70-200/2.8 II, so very fast. From what I heard of S. 135/1.8 it shoul be similar.
On IBIS bodies I do not find OIS too beneficial with 135mm. It is fully useless for fast action. For portraits IBIS is fully sufficient simce we need times of 1/50 or shorter to eliminate motion blur.
And IBIS allows up to 1/10s of reliably sharp images. How longer times are needed for regular usage of such lens? How longer times OIS allows? Maybe 1/4s? Is the difference worth that massive price difference?
Just my thoughts for OP's consideration.