Both TV's are great, if you can, the ST60 is a better option because it does have a higher light output than the ST and GT50. Do not listen to comments about neither being an option in a well lit room from people who haven't seen them in well lit rooms either, they all three have excellent louver filters and are all the same filter on the ST50, GT50, and GT60 Its less about the amount of light than it is about the angle. If your TV will be against the same wall the windows are on, thats an ideal situation because the biggest issue is when light hits the TV from a particular direct angle causing the colors to wash out a bit, i have my GT50 in the living room in New Orleans with direct sunlight year roung coming in two floor to ceiling old school windows that arent filtering any light at all and thin white silk curtains that are again not at all efective at eliminating light and are strictly there for privacy. The tv looks excellent at all times of the day even when light is hitting the screen at certain hours because its never coming from whatever angle it is that causes an issue. In fact, the Samsung UN55ES7100, a top of the line LED backlit LCD TV from their 2012 lineup that i just bought in the 55' size for another room and follows the increasing trend of making LCD TV's with glossy screens to help achieve higher contrast levels than matte screens has much worse reflectivity than the the GT50. They put out plenty enough light, and the filter will take care of all but the worst lighting issues, if you are worried, i encourage you to go buy a GT50 or ST50 or ST60 from best buy and put it on one of the brighter settings and you'll see for yourself its fine or you can bring it right back.
As far as THX bright mode and ISF settings, I wouldnt worry yourself, I had similar concerns and am now looking at a ST60 as well and considering selling the gt50, its getting down to petty differences at these levels, but when making a purchase like this thats your right and its just good practice to be as informed a consumer as is reasonable. Striclty speaking in plain fact, the ST60 has better picture quality than the GT50 is capable of displaying, its not huge but its there, you would never know or be dissatisfied if you had a GT50 but if you are making a purchase and have both options available then by all means get the ST60, the only reason to get the GT50 above the ST60 would be if you are a avid gamer that needs superb response times as the ST60 has somwhere around 20ms greater response lag in game mode as compared to the ST50, to most its a non issue but to some it can be worth sacrificing the extra picture quality.