What to do after 145?

I suggest just getting 2:1 ST and demo. Leveling up one of the two means the other one suffers, which isn't as good as leveling both of them at the same time.

Edit: Y U HEK!?

Having less than ~20 Demo is pointless though, so you're best off getting the first 20 points in it in one block.

Think about it; why have 20 ST and 10 Demo that you're not going to use, when you can have 30 ST (more periodic Haste? -shrug-) or 10 ST and 20 Demo that you are going to use?