Not sure what the difference is, or if it's even good.

The #-way combo is how many shots in a row that you string together. The other combo number is how many combo's throughout the game you put together.
For example, if you started off with a 4-way combo, that would count as 3 combos. then later you have a 2-way combo, you now have 4 combos.
Once you get to 10 combos they are worth more for the rest of the ball. there is even a multiball at 25 ...but I haven't gotten that far yet

Hope that made sense...

I think the combos are cool on SM, but I don't really go for them all that much. I naturally try for the white arrows because I get two modes going in one game if I'm lucky.
I still like the excitement of finding the secret combos in some of the William's games