This is right down my alley.
I was in your same shoes in the past few months and I went with the Samsung PN51E550. Now, I ended up exchanging it for a Panasonic 55ST50, but for the time I had it, the Samsung was a great TV. I didn't exchange it because of lackluster picture quality, but rather I had an opportunity to get a better overall TV for $50 less.
Initially, I chose the Samsung over the Panasonic because it was a more fully featured set. If comes with two pairs of 3D glasses and built in Wifi. The built in Wifi is not only handy in accessing streaming entertainment services such as Netflix, Amazon Instant, HBO Go, etc. but also in getting software updates over the air and keeping your set up to date.
In terms of picture quality, it is very close depending on what your preferences are. The Samsung, to me, has a very natural looking picture in movie mode and colors are very pleasing to look at. It does though, have disappointing black levels in comparison to any Panasonic. In my experience, as well, is that the Samsung doesn't handle motion as well as the Panasonic either. I haven't tried 3D on my Panasonic yet, but I really enjoyed the Samsung in this respect. If we're strictly discussing 2D picture quality, I'd say the Panasonic is the superior set.
If I were choosing between the 50UT50 and the PN51E550, I'd pick the Samsung again because of my needs. If I were in your shoes, though, maybe you should look at the Panasonic U50 instead and save at least $100.