Unless you need lower power draw i would vote for the st50.
The et5 appears to be an average run of the mill edgelit led tv which means it's not likely to do very well in darker scenes, and it's very likely to have poorer contrast.
Due to it being edge lit the panel uniformity (eveness of brightness across the screen) can only get so good and the plasma will have near perfect viewing angles where as the led will lose contrast off angle. (though ips panels like the one in the et5 perform very well in that area.
Plasmas usually have a higher motion resolution as well and the st50 is a very highly praised plasma: http://reviews.cnet.com/flat-panel-tvs/panasonic-tc-p50st50/4505-6482_7-35117941.html?tag=contentMain;contentBody;2r where as the et5 hasn't been reviewed as far as i can tell.
As far as lifespan goes they are pretty much on par now, there isn't enough information out there to say for sure which will last longer, both are being rated at upwards of 100000 hours (30 years) to half brightness.
Leds don't usually suddenly fail but it is possible: https://en.wikipedia.org/wiki/List_of_LED_failure_modes
Plasmas are less susceptible to sudden failure of the light source but it's not likely for either to suddenly fail.
Plasmas do draw more power but the power supplies are probably picked with that in mind to decrease the failure rate, and if good quality power supplies are used then the supply isn't likely to take any other components with it when it goes and can be replaced.
The plasma has an anti reflective coating as well, so while it may be more reflective it won't be like old tube tv's for reflections.
There is still the slight chance of burn-in on plasmas but it isn't something to worry about unless you only watch 4:3 content or 16:9 content with black bars. If you will be alternating content that fills the screen and some that doesn't and you don't leave static images on the screen for days at a time then plasma will do you just fine (i've used a 2008 panasinic as a computer monitor for a few months with only slight taskbar image retention that went away with some movie watching and gaming).
If your interested in led sets over plasmas you should look into a good local dimming set. Local dimming sets dim parts of the backlight for a much higher contrast ratio, and there are edgelit models out there with local dimming that can be had for very reasonable prices.
55" sony kdl-55hx850
http://reviews.cnet.com/flat-panel-tvs/sony-kdl-55hx850/4505-6482_7-35118056.html?tag=contentMain;contentBody;1r
65" Samsung un65d8000
http://reviews.cnet.com/flat-panel-tvs/samsung-un65d8000/4505-6482_7-34513689.html?tag=results;prodInfo
55" Samsung un55b8500 an older one would be a good deal if you can find one used.
http://reviews.cnet.com/flat-panel-tvs/samsung-un55b8500/4505-6482_7-33740174.html?tag=results;prodInfo
Lg infinia 55le8500
http://reviews.cnet.com/flat-panel-tvs/samsung-un55b8500/4505-6482_7-33740174.html?tag=results;prodInfo
55" Vizio xvt55vs3
http://reviews.cnet.com/flat-panel-tvs/vizio-xvt553sv/4505-6482_7-34122871.html?tag=results;prodInfo
Most of these come in smaller sizes as well, good luck
The et5 appears to be an average run of the mill edgelit led tv which means it's not likely to do very well in darker scenes, and it's very likely to have poorer contrast.
Due to it being edge lit the panel uniformity (eveness of brightness across the screen) can only get so good and the plasma will have near perfect viewing angles where as the led will lose contrast off angle. (though ips panels like the one in the et5 perform very well in that area.
Plasmas usually have a higher motion resolution as well and the st50 is a very highly praised plasma: http://reviews.cnet.com/flat-panel-tvs/panasonic-tc-p50st50/4505-6482_7-35117941.html?tag=contentMain;contentBody;2r where as the et5 hasn't been reviewed as far as i can tell.
As far as lifespan goes they are pretty much on par now, there isn't enough information out there to say for sure which will last longer, both are being rated at upwards of 100000 hours (30 years) to half brightness.
Leds don't usually suddenly fail but it is possible: https://en.wikipedia.org/wiki/List_of_LED_failure_modes
Plasmas are less susceptible to sudden failure of the light source but it's not likely for either to suddenly fail.
Plasmas do draw more power but the power supplies are probably picked with that in mind to decrease the failure rate, and if good quality power supplies are used then the supply isn't likely to take any other components with it when it goes and can be replaced.
The plasma has an anti reflective coating as well, so while it may be more reflective it won't be like old tube tv's for reflections.
There is still the slight chance of burn-in on plasmas but it isn't something to worry about unless you only watch 4:3 content or 16:9 content with black bars. If you will be alternating content that fills the screen and some that doesn't and you don't leave static images on the screen for days at a time then plasma will do you just fine (i've used a 2008 panasinic as a computer monitor for a few months with only slight taskbar image retention that went away with some movie watching and gaming).
If your interested in led sets over plasmas you should look into a good local dimming set. Local dimming sets dim parts of the backlight for a much higher contrast ratio, and there are edgelit models out there with local dimming that can be had for very reasonable prices.
55" sony kdl-55hx850
http://reviews.cnet.com/flat-panel-tvs/sony-kdl-55hx850/4505-6482_7-35118056.html?tag=contentMain;contentBody;1r
65" Samsung un65d8000
http://reviews.cnet.com/flat-panel-tvs/samsung-un65d8000/4505-6482_7-34513689.html?tag=results;prodInfo
55" Samsung un55b8500 an older one would be a good deal if you can find one used.
http://reviews.cnet.com/flat-panel-tvs/samsung-un55b8500/4505-6482_7-33740174.html?tag=results;prodInfo
Lg infinia 55le8500
http://reviews.cnet.com/flat-panel-tvs/samsung-un55b8500/4505-6482_7-33740174.html?tag=results;prodInfo
55" Vizio xvt55vs3
http://reviews.cnet.com/flat-panel-tvs/vizio-xvt553sv/4505-6482_7-34122871.html?tag=results;prodInfo
Most of these come in smaller sizes as well, good luck
