I always used the 90/6.8 Grandagon believing it was as sharp and good as more expensive, heavier lenses. But after I tried a 90/4.5 I swore I'd never go back, weight and size be damned. The larger coverage wide angles (Schneider XLs, the fast Grandagons) aren't any degree sharper BUT they are much more even. Skies finally look like how they should - not vignetted. Plus focusing is easier and the extra coverage never hurts.
It's hard to fault the 150 Rodenstocks, provided you have a good example.
If you are investing that much, it seems like you should be looking at a third, longer lens. Perhaps a nice 240-300mm if the Ebony can handle it?
Frankly, with base tilts (Wistas and others), I fail to see why someone needs assymetrical tilts for horizon line landscapes or fauna shots. For product shots (pointing down with a vertical back) they are nice to have, as well as some architectural swings, but otherwise WTF?