Well there's also the part where physical maturity (as in peak physical skills) are different for position players and pitchers. Sanchez's velo will likely (naturally) decline a modicum each year from here on. So, in terms of physical maturity, those are equivalents.
Mental maturity, sure. The vast majority of 17 year olds don't know s*** about s***.
But, all of that is rolled up when you age-adjust minor league numbers (a foundational component of any projection system). With these things considered, Sanchez's minor league numbers are worse than Gomes'.