Generally 1 tablet equals 1 teaspoon. Unless you've got double strength rennet, in which case 1 tablet equals 1/2 teaspoon.
As far as what type of rennet, that's a more complicated question, involving how many vegetarians you know, how sensitive you are to the bitter rennet flavor, how you feel about GMO foods (though I think very few home cheesemakers use GMO rennet), and how old-world and traditional you want to be with your cheesemaking.
Personally, I have only used double strength vegetable rennet thus far, but next time I purchase, I will try calf rennet, to see if there is any improvement in flavor.