I asked basically this question directly to one of the video reviewers (Working here has to have some perks!) with the caveats that it will be used for 95% HD/SD broadcast material, in a bright to dimmed (but not blacked out) room and a £750 max budget.
It was the 350 that had caught my eye, but the answer came back that whilst the 350's blacks aren't as good as most other plasmas, they're still better than LCDs for money, but the 590 "..has great performance, decent blacks, THX preset and ISF calibration controls so it should meet all your criteria.
" In other words, worth the extra in performance terms.
FWIW, it was mentioned that the Samsungs offered better blacks but suffered image retention which I felt would bug me senseless like dust blobs on my PJs LCD panels and Panasonics offered the best blacks but the 50Hz issue is an issue for someone who won't be gaming or watching that many DVDs/Blurays on it.
As someone who's more fussed about sound than picture, but can't resist having a tweak anyway, the THX preset seems universally agreed to be very close to spot on and a good safety net if I get too far out of whack whilst trying to better it. In other words, a proper tweaker will get much the same performance from both, but the 590 is closer out of the box.
In the end, the (not specified) criteria of viewing photos stored on my NAS over the network swung it for the 590, coupled with the fact that I too would end up thinking 'what if?', whilst the 990 was to far out of my price range to create a worry.
Mine arrives the day after tomorrow. Not bad considering I only decided I wanted a panel at all last night.