I've heard good and bad things about Square Trade but they will likely get you the phone faster and they have a local repair option, but if they send you a refurb it's a shot in the dark as to the quality of it. That's a big issue with Asurion (the carrier insurance people), they send complete junk refurb phones out a lot.
With Samsung it will take longer to get the phone back, up to 2 or 3 weeks, and they will always try and fix it before they just replace it. However if they replace it you are getting either a Samsung refurb or a brand new S6. You can also only do 3 'damage' claims in 2 years, not sure if square trade has any sort of claim limit.
I went with Samsung since it's the OEM but both Samsung and Square Trade are pretty close to the same feature wise.