They are similar in that they are both speedy infielders who aren't good hitters.
Sadler never really had a defensive position, while Hamilton is a sort of okay SS who could be a rangy second baseman. Hamilton is only listed at 5'10" but that's 4 inches taller than Sadler.
Hamilton has stolen 122 bases in 139 tries in the minors, while Sadler was 171 for 224 in his 12 minor league seasons.
In his 220 minor league games, Hamilton has an OPS of .750.
Sadler got attention with an .836 OPS in A ball at age 20, but followed that up with .715 in AA and .621 and .669 at AAA. He got to the majors for parts of 8 seasons and 418 games, so maybe Hamilton won't be able to match that.
A better old time Red Sox comp for Hamilton might actually be
this LH second baseman who stole 208 bags in the majors but only ever had an OPS+ better than 88 in one season, where he got to an even 100. Hamilton probably won't even match that guy's career .327 OBP though, and likely won't play anywhere near as many games in the majors.
If things go well, I could see Hamilton eventually
putting up similar offensive numbers to this switch-hitting 2B, (.258/.327/.341) but as a former college player, if he does make it, he almost certainly won't play as many games or years in the majors.
And if things don't go well there's a real likelihood of Hamilton never playing much in the majors at all. Maybe a season or two as pinch runner type of bench player.
On the other hand, while Sadler has the same last name as this guy...
View: https://www.youtube.com/watch?v=8kj9qv6rmG8
Hamilton has the same name as a musical that was fairly popular.
View: https://www.youtube.com/watch?v=VhinPd5RRJw
Point Hamilton.