Out of nowhere, during a visit to my local Costco, I saw the last remaining 60ST30 for $900....
That is an Outstanding price for a 60" ST30!
The cost difference between the two panels could make this a difficult decision, depending upon your financial preferences: There is little doubt that the ST50 is a Better Set than the ST30. But is it $800 better? What other usage could you / would you put that money toward?
Personal experience is that the 60" ST30 provides a wonderful picture, at a 9ft viewing distance (or from farther away), absent Green Blobs or Pink Tint.
If your ST30 lacks either of these flaws - or you are so blessed as to NOT see them! - then I would keep the 60-inch, UNLESS your viewing distance is closer than 8 feet. (Larger panel shows the inherent picture flaws - of your source material - more readily than smaller panel will. 9 feet from the 60" is great for Blu-ray, so-so for most "HD TV" programs, and Too Close for most SD DVDs...)
The GREEN on our Nov Build ST30 was easily the LEAST invasive of any of our earlier 2011 panels (most people would not have noticed the "Green Blob" on that panel without having it pointed out), AND it got better as the panel aged, finally fading away to insignificance after several hundred hours of usage.
At the same time, apparent Black Levels and Contrast improved as the panel aged.
Alas, the PINK TINT on that panel grew WORSE as it aged.
Note that a) Not everyone "Sees" the pink - or, at least, is Bothered by it;
b) the Pink appears to have been a problem on a lower percentage of panels than the Green - and a number of owners have reported that the Pink on their panels actually IMPROVED over time, much as the Green tends to do; and
c) Even at its worse, the Pink Tint IS highly Content Variable: some material shows it off, most material does not. "Dirty Whites" or Light Grays display the pink "best": B&W movies; "House" -- pink likes the off-white lab coats, and also much of the "institutional" wall colorings used in the show; off-white clouds / steam / fog / mist / water spray - and, of course, these must appear on a part of the screen that is impacted by the Pink Tint in the first place.
To test, find content similar to those mentioned, pause on several scenes where this content appears ALONG THE EDGES of your panel (particularly the Right edge, but check all four), and see whether you can notice a location-specific change in color (ie, from Off-White to Pink...).
If you don't, you may be home free. If you DO, then the ? becomes Will it Fade, Remain the Same, or Grow? Owners have reported all three results, and only time will tell you the answer.
(No. 1 Son has a 2011 Panasonic that started out with green blob and a horrible pink, BOTH of which faded over the first several hundred hours - so it CAN happen!)
Upon re-reading, above sounds rather negative on the ST30. Put into perspective, we tried 5 2011 60" Plasma sets: 1 arrived with a cracked screen, all the others had Green Blobs AND Pink Tint - so am afraid I am somewhat jaded. (Tried a pair of S30s hoping to avoid the Green: first one was cracked; 2nd had both problems. Would have given up but for talk out of UK that Panasonic had released a "Firmware Update" to address the problem - and found a Great Price at Costco!
Turns out that Update had mostly Disappointing results.)
That said, the Nov Build ST30 was easily
the best of the panels, OOTB, for overall PQ - I could "find" the green, but the panel did NOT show Green Faces, which was an issue (!) with the earlier sets.
As the panel aged, and the green faded, the PQ was mostly impressive, esp with true HD material (Blurays or HDDVDs).
Alas (again), the Pink Tint grew in area & intensity, finally becoming Just Too Obvious to ignore - so the panel is now gone. (At the end, Pink Tint covered nearly 1/3 of the screen, encroaching from all 4 edges, and was becoming visible in a wider range of content - although still not "universal" - Bright or Dark scenes still hid it.)
Most of 2011 sets did not have THIS much trouble with the Pink, & hopefully neither will yours (if you do decide to keep it). Best of Luck.