The JVC vs. Sony is always going to be a hard call, for me the sharpness of the JVC is more why I went with the JVC over the Sony rather than black levels. Not that last year's Sony hw30's are not relatively sharp, but it doesn't resolve pixels as finely as the JVC's do. The extra blacks did influence my decision slightly, but it wasn't the primary reason. Basically based on last year's comparison (and I'm not sure it has changed much, maybe), the Sony wins at color accuracy and motion and gaming and 3D. The JVC wins in sharpness and black levels. The Sony spanks the JVC in 3D.
Even being a JVC owner myself, I think going by only black levels is a so-so argument to overrule the lower-end Sony with the lower-end JVC (RS-46 vs. hw50). The blacks on the Sony aren't quite JVC territory, but they are still very respectable and not DLP territory either. The added blacks on the JVC will surely be noticeable in some content, but for most buyers I don't think the black levels of the Sony vs. JVC are the deal-breaker. If I had to do it over again, would I still have bought the JVC RS-45 over the Sony hw30, maybe, but I'd have to see them side-by-side in my own home to know for sure (and showrooms don't count, too hard to tell when PJ's are this close).
If I had to guess, I'd probably still take the JVC due to its sharpness (but keep in mind I'm a sharpness junkie), but if Sony's are a little sharper this year, then man that is really going to make this a hard call, but I'd lean towards a Sony in that case (but that is personal preference and it could go either way). I really like the way the Sony's calibrate better than the JVC's color, but more so you get better 3D and gaming on a Sony.