Adam,
It's almost two stops slower, not one stop, so that may make a difference to you.
By f8 or so (for sure, possibly by about f5.6), they will both probably be very similar in performance, but the way they render out of focus areas may be slightly different. One thing I have noticed between the older lenses and the newest Leica lenses is that I prefer the way the older lenses transition from in-focus to out-of-focus. It just seems more graceful in that transition in the older lenses. Thankfully, it also means I generally prefer the much cheaper older lenses than the newest versions of the same lens.
Regardless, from about f8 through to the smallest apertures, they will probably appear very similar in performance.
So the only real question is what happens from about f8 to the largest apertures, and is that difference important to you?
You gain almost two stops with the summilux, and that is nothing to sneeze at. You also gain the ability to reduce the depth of field considerably with the larger maximum aperture of the summilux. The summarit is going to be sharper it it's maximum aperture than the summilux at the same aperture (probably), because it is a bit better corrected.
If you were a pure landscape shooter in the traditional manner, I would think that these aspects would be of little importance, but for documentary shooting, the ability to isolate with the summilux may be worth the price of admission, even though it does come with some compromise in absolute sharpness in the image at the largest apertures.
I made a comparison between a summilux and summicron at f1.4 and f2 (the two maximum apertures) and a bit stopped down. By about f4 and certainly by f5.6 they were very close, as to be nearly indistinguishable. Now, they were contemporary, and the summarit is new, so I expect the differences to be greater. If I still have the comparison images, I'll post them tonight.
Only you can determine what is the more important factor, but for my purposes, I decided that the summilux was the better choice for my shooting, even though in many cases I shoot at f5.6 or f8. When I need the f1.4, I have it to use.
---Michael