I'd take the UT50. If your V10 is calibrated, then the UT would have less options as far as adjustment. It's been the general consensus here that the UT50 is essentially the same as the ST50 minus the AR filter, which shares a panel with the GT50, which is not far apart in performance from the VT50...you get my point. Considering the age of your V10 (a 2009 model) combined with the known issues from that model year (the dreaded black-level rise), I'd say the the UT50 should generally have better performance than your V10, the only caveats being the UT only has two HDMI inputs and is very reflective, which you indicated wouldn't be a real issue for you. Considering the prices that 55" UT50s are being blown out for now, I'd say it's a no-brainer, especially when you factor in the fact that Panasonic will not be offering anything at the price/performance ratio of the UT50 ever again.
ht Panasonic TC-P60ZT60, Monitor Audio: Silver RX6, RX Centre, Radius 90HD; Martinlogan Dynamo 700, Marantz SR5006, PS3, Oppo BDP-103D
2ch Marantz PM8004, Sony BDP-S1000ES, JVC T-X3 tuner, Monitor Audio Silver RX1, REL T3, Apple TV, Peachtree Audio DAC•iT
lr Panasonic 50ST60, Sony BDP-S5100