Sano is a much better player. On offense, Sano has a career wRC+ (probably the best single indicator of offensive output available) of 118. Vargas had a career wRC+ of 100, exactly league average. On defense, it's harder to gauge because Sano has so little time at first base. He was -5 DRS last season and his UZR was pretty equally bad. It was also Miguel's first season at first. FWIW, in a microscopic sample size this season he's zeroed out (average). Vargas was decent at first, posting a +4 DRS in his career. Time will tell how that shakes out but I expect Sano to ultimately grade pretty similarly to Vargas in that regard. For a first baseman, Sano is pretty athletic and mobile enough to do okay over there. As a whole, Vargas posted an fWAR of 0.9 in ~850 PAs while Sano has an fWAR of 8.8 in ~2300 PAs. That's not particularly close; Vargas is marginally above replacement level while Sano is somewhere around average MLB starter in overall value.