She's more ready than McCowan in terms of offensive skill, but McCowan is a better defender and rebounder at this point. The main reason I'd take Tiera at 1 is that she is so big and strong and has incredible hands. She is just scratching the surface of her potential. Once she hits the pros, she's still going to be bigger and stronger than everyone besides Griner, Cambage and Fowles. A midrange shot can be developed with time, although I don't think it's necessary for McCowan right now. Brown isn't as physical as Tiera, although she does have a great frame and a more developed offensive skill set that should transfer over to the pros.
If Indiana gets McCowan, she'll be good for 10+ rebounds per game right off the bat. I don't think you get the same result with Brown.
McCowan's red flag is her offense, which right now consists of finishing putbacks and burying opposing posts inside to get good position and hit layups. She also has improved immensely offensively from a year ago and is just scratching the surface of her potential, os that isn't as big of a concern to me unless it appears she regresses next year without the 4 seniors on the court with her. The other potential red flag is attitude, in that she was pretty rude during an interview after the title game (understandable that she'd be upset, especially after missing the layup late that would've put Mississippi State up by 2) and she's not afraid to get chippy out there, but that's minor to me considering most of the greats are known for being a little chippy out there or getting T'd (Leslie, Jackson, Taurasi, Cambage, etc.)
Brown, to me, has bigger red flags. Kalani Brown faced very few quality bigs last year (Thome, Billings and Gulich) and was badly outplayed against the best opposing big she faced last year (Gulich). The other red flag is that she regularly looks winded running up and down the court in games. WNBA games are faster paced and Brown is going to need to be much better conditioned if she wants to log solid minutes as a pro, otherwise she'll be a liability out there.
Right now I think both will be 1-2 in the draft, but a few players could surpass them with big seasons.