You actually asked a couple questions. Best for roughing blanks is probably a 3/4"X4 tpi. Best general purpose is, IMHO, a 3/8"X6 tpi. The 3/8x6 will do real well for a lot of cutting, even thick roughing.
But, there is a flaw in the reasoning of your question. A one blade purchase is bad economy. You will need and want more than one. And Suffolk Machinery (the Timberwolf people) usually always have a 3 for 4 special going on and shipping for four isn't much more than just one. If the budget can handle it, I suggest one 1/8"(I forget tooth count); two 3/8"X6 and one 3/4"X4.