15% LUK Cape or 10 ATT?

The cape would give you a bigger damage boost, yes. 11.5% for the cape vs. 5% for the dagger scrolls.
In terms of "investment" you'll have to talk to merchants about resale values, etc. I don't know a thing about that.

Also, if I may ask, what cape are you wearing now? If you have a PAC with a few attack on it, or something scrolled with more than a handful of LUK, might need to redo the math to include the damage lost when giving up the old cape.

I ignored the cape stats in my stats I wrote above, so that's all fine.