I think the key factor tonight (or tomorrow) will be ITV's error in leaving the 15/20 minute overlap. It will most likely hand the BBC the PR victory especially when BGT is likely to see a decline from its equivalent episode last year (and I'm guessing that open the door to "lowest since 2007" kind fo headlines). The 15/20 minute overlap and general decline for ITV talent shows should see to that - the weather might not help as well although sunset is still quite early.
I think BGT will edge it on averages though I'll not be surprised if it's by a narrow margin or if it goes against them. Nonetheless, ITV and Syco tend to be hopeless at spinning ratings - so I'd expect the BBC to run away with the PR battle tomorrow and then ITV to release a limp statement on Monday afternoon saying they're pleased with the ratings!
I don't even think Cowell commenting on TV matters at this point - it's not as if it is short of publicity anyway and with the press launch it was inevitable that he would say something (and I see will.i.am is todays DS headline for his latest "attack" on Syco/XF/BGT). The perception is certainly that BGT is on the ropes and TV is the next big thing. ITV have definitely be flat footed here - that the Cowell/Walliams interview is airing an hour AFTER the premiere kind of sums it up.
I don't think something in the low to mid 6's is acceptable for The Voice, tbh. That's not to say that the show is dead if it only manages 6.5m, it may well get a second week bounce, but as a premiere rating it's rather average given the momentum just now. I'm not sure there's any realistic prospect of ITV getting a great outcome tomorrow (which I'd probably class as BGT being about level or up and TV under 7m) but if they can get 9m (exc +1) and hold a 1m lead then I think there will be some relief at least. Harry Hill will likely depart on a low note although that might not bother ITV too much! D.M.N's predictions would probably keep everyone happy (although the BBC would probably be happier with that than ITV).
The BBC have certainly won the mind games. But after weeks of build up, there will finally be a result tomorrow. Although I doubt that will be conclusive either! Sundays should be interesting on here for the next couple of months!