Weiss: "So, I've got to do a Threesome, with you and Ruby?"
"Is there a problem?"
Weiss: "Not really…"
"Look I don't like this either, but we gotta do it, for Ruby, okay?"
Wiess: "Fine! But this will not change anything!"

This really shouldn't be tough choice:

Weiss:
+Rich
-Alpha bitch.
-Racist.
-Couldn't defeat a random chainsaw wielding mook.
-Sucks at card games.
-Asshole family.
-Not what the fandom expected.

Penny:
+Cute.
+Robot girl.
+Combat ready.
+Cute.
+Dangerously adorable.
+Red head.
-Dead.
+But can be rebuild.
+Did i mention cute?

Clearly the answer is obvious. She should pick Weiss.