First of all, great choice in looking at the plasma models from Panasonic. They are currently the best plasma displays out there and in my humble opinion the best displays you can currently buy. The soap opera effect takes the Elite out of the running for me.
Specifically to your question, the top of the line sets are the ones that come closest to the Pioneer Kuro in terms of the black level test where the TV reviewers were brought into a black room and the TVs were already on and they couldn't tell. Here's the criteria I would use:
1) If you have the cash and
2) You don't have a need to get another TV within the next 5 years
Then yes, I'd say go for the higher end model. The only caveat is that the TVs right now are getting larger and the price point is going down and the tech is likewise trickling down. So, waiting next year, you may be able to get a 70" model in that line or a 65" that has a better picture at around the same price point.
If I'm to take this on my end, I would look seriously look at the ST60. It offers spectacular picture quality at a better price/value. You might want to read Tom Norton's review of the ST50. His praise speaks volumes and he's the one video reviewer that I trust implicitly with his reviews. His opinion is worth gold. Here's what he concluded:
The hardest part of doing this review was finding something to complain about. I wasn’t very successful. With a remarkable price, superb 2D performance, and 3D that can only be bettered by spending a lot more (and often, not even then), this Panasonic is a no-brainer. When I tested three different Panasonic plasmas last year, the TC-P55ST30 was my sentimental favorite. And while we haven’t yet tested Panasonic’s premier 2012 offering, the VT50-series, if your budget won’t stretch that far, it’s hard to imagine you not being thrilled with the TC-P55ST50.
So, that's the way I'd approach it.