rare or craft 10fcr/dual leech.

But one of the rings can have ED%, and that helps a lot ofc. Still you want one leech source...

I think depending on the rest of your build mainly. If you need two rings to achieve 125 fcr, you must always go for 1 dual leech ring, and u can go for sling if u want mf on the other side. If you only need 10fcr, the other must go SoJ/Bk5

nice are slings worth it over 2 10xfcr?

1 sling + 1 fcr dual leech is nice

Here is my planned pvp build, change gloves into LOH and add a 20fcr diadem and ur set:
https://maxroll.gg/d2/d2planner/0idwz0ju

Also add lvl 1 sigil.

Or just make ondal's build.