I am new to AR15s and really to guns in general, so I recently found a video discussing pros/cons between MLok and Keymod. They suggested that one is likely to overcome the other in market share, and that since MLok has been adopted by companies such as Magpul, it is more likely to succeed. So if that's accurate, I'm glad S&W is a adopting it here. The additional length(I think I counted an extra 6 rail positions vs the quad rail)
looks better to me, and I would assume makes shooting easier when you can grip the handguard further out. Really excited to see this in person!
Found this website today. No idea if it's a reliable note, but it suggests these might be available in the next few days.
S&W M&P15T 5.56 SA 16B 30R - Sam's Locker