I have a #9HP and this is a very easy rod to cast. I usually use it on a boat, and it will throw 30' of leadcore, or a 300gr line easily. The tip isn't especially heavy, but I think that the taper progressively lodes the rod so that it doesn't feel like it is collapsing even when it is overlined. The shorter lenth is not a concern on a boat, but it makes the rod feel very light, which is nice if you are false casting all day. I like this rod.
I don't have an #9 sts but I do have a #7 & #11, and I have cast the #9. These are terrific rods and maybe better all around. They have a very possitive feel. Comparing the two lines, I think that the STS could be called a higher performing rod(stiff butt, generates lots of line speed), but the HP is sweeter(fuller flexing, easier to cast).
If you cast them side by side, the STS will feel like a better rod, but the HP might be better for some.
There are so many nice sticks out there.