Both will perform the same in a bright room. Same filter, same brightness capability. The GT50 has the benefit of having more modes which will give you better options for having separate settings for day/night viewing. You'll have to use Cinema and Custom on the ST50 where Cinema mode has a slightly lower gamma.
The VT50 has a better filter (but also costs more).
It's the reason I got the VT50. It's actually pretty amazing compared to other TVs. I have a wall of floor to ceiling windows behind me & a sun room next to that. TV does great. I do shut the blinds if I really want to enjoy my viewing (geek out). However you can easily see the image during the day. The TVs ability during the day to keep the black areas look dark is amazing. You can still see good amount of depth even during the day.
I'm sure the GT50 which you ordered will be excellent as well. If it's not enough give the VT50 a try
The VT50 is only slightly better at handling reflections and darker blacks in a bright room. However, the over all image appears slightly dimmer in all modes. I have owned both and returned the VT50 to return to the GT50.