Sanchez's swinging strike rate hasn't been as ridiculously filthy as Osuna's, and xFIP is misleading for extreme FB pitchers.
SIERA is always the most accurate, if only marginally. 2.74 SIERA for Osuna vs... well, there are no SP/RP splits for SIERA or SwStr. That's stupid. Sanchez's was a fantastic 2.53 last year, though. So considering this year if we call it a near-tie, I'll take Osuna's 14% SwStr over Sanchez's 6.5% last year, although the ground balls add value.
tl;dr near tie, but I'll take Osuna. As an asset, purely as relievers, I'd take Sanchez because he didn't have arm issues last year