Walls and Ceilings - Help with material decision...

Thanks in advance to anyone that can help me with this.
I'm finishing my basement and am at the point of putting up a ceiling. I was going to go with track/tile, but I just finished wainscoting the basement and (to my amazement) it looks beautiful. The grain/stain look against the painted drywall (purples and greens) looks so great that my wife and I don't want to detract from it by having a non-natural look for the ceiling. All the pipes, wires, etc. are in the joists and the ceiling is low enough that I don't want furring strips (I want to go flush to the joists). I plan on using 4 x 8 sheets of something, with 2" trim over the seams. That way if I need to get into the joists for plumbing, etc. I will just have to pull down the section with the problem.
But I can't fgure out what type of 4 x 8 sheets of wood to use. Can't be too thin, can't be too thick. I need a finished, grain side for looks. I need something that will not bow over time (although they will be supported with screws around the perimeter of the sheet and nailed in all the joists in the middle). Any suggestions from the pros out there?

You can buy 4x8 sheets of bead board in 3/8" thickness - a little thin may sag over time. RB&B and T1-11 both come in 5/8".