High school teacher opinion:
Chalkboards that are worth a darn are hard to come by these days. If you can find a real slate board, grab it. The green boards are just a special paint. IF you can get one just like schools have, it's OK, but don't get a cheap one, they don't erase well.
WHite boards are my favorite because you can have several bright colors for drawing different levels or lines or whatever.
As long as you have: an inexaustable supply of markers (about 20 times the number you first think you are going to need--They dry out so fast); you must NOT use those infernal erasers (They are only good for a day or two, then must be thrown away. Instead get a box of paper "shop towels" from an auto supply. They look like heavy duty napkins. Much better for erasing).