Pex or copper for return

Which would be better, PEX or copper in the return of the slant-fin Base Board . I'm concerned about securing the PEX inside the S/F BB.Where or how do you secure it inside the BB.

For the same size pipe/tubing, pex has a smaller inside diameter, so take that into consideration.

Securing in S/F base board

I am wondering about securing it in the base board. If I use PEX will the expansion (PEX suppose to expand when hot), be to much inside the S/F BB

I don't know the relative expansion rates for copper and pex, but they both expand when heated. As to running pex along the baseboard, two thing: will sunlight strike it (not good for extended periods), and will it be protected from potential knocks and cuts (copper will dent, but is much harder to cut)? I'd lean towards copper for the increased flow, impact resistance, and UV tolerance. Pex would be cheaper and require fewer joints.