ST50 v.s the UT50 What is the $300 differences?

This question has been asked a thousand times here, the search feature is your friend. But I'll answer basically AR filter, custom controls and an extra input. If your room is light controlled it is probably a wash.

The St uses the same panel they used on the VT last year (infinite black panel pro I believe)

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Pretty darn close from what I have read, the advantage for the ST would be in a brighter environment, the black level holds up much better. The entire line from U to VT has outstanding blacks this year.

Thats incorrect all the panels are new this year and designed from the ground up in fact the ST blacks exceed the VT blacks from last year.

You're confusing the filter with the panel. It uses the same filter as the VT30, but the panel is new as chunon said. The UT has no filter, the ST/GT have the infinite black pro filter and the VT has the infinite black ultra filter. All of the panels are new and have the increased brightness and better black levels (among other things).

You can calibrate grayscale on the UT50, but you have to do it in the service menu. The ST50 has them in the user menu, plus it has a gamma slider which is really useful.

Other differences that the ST50 has that the UT50 lacks:
A third HDMI input, rotating pedestal stand (for the 50" model), a simple web browser app in the Viera Connect internet apps.

Well said

One other difference would be if you want a 65". ST only.

Do you mean switchers? Yes there are many models of HDMI switchers such as 4 in and 1 out for example.
Monoprice has a good 4 port HDMI switcher that also can output separately the digital audio to optical or coax to an older non-HDMI receiver.

regarding extra HDMI inputs if you have an AVR you really only need 1-2 HDMI inputs on the TV itself... for example I have a Sony HT-SS380 and it has three HDMI inputs on the receiver/amp and a single HDMI output to connect to a HDMI input on the TV

I can then use three sources via the receiver/amp and use the remaining HDMI inputs on the TV for other devices... and if you have a AVR with 4-5 HDMI inputs, you really only need a single HDMI input on the TV.

the only issue with this setup would be if you need separate settings per source device (but that is rarely the case on well made HDMI equipped source devices)