For iPhones, the benefits and costs AC/ST are similar. People keep saying they would rather deal with Apple - you can with ST, so AppleCare is not really an advantage in that regard.
I have owned both ST and AC over the years, and choose based on current pricing, available promotions(ST), deductible for device replacement, max number of replacements, and whether or not the third year is of interest. Been very happy with experiences with both.
For the Apple Watch, Squaretrade does not appear to have an appropriate offering yet. They only have a generic plan that tops off at $499 in max value and costs $104.99, albeit for 3 years. Of greatest concern, it appears you can not just bring an Apple Watch to Apple for same day repair - they just seem to guarantee 5 day service. AC+ is just $69 for two years, includes genius bar service, and allows for two device replacements (at $79 each). It appears ST does not have a "co-pay" for device repair though.
So while I am a fan of ST for iPhones, until they have a more specific Apple Watch program, I would stick with AC+.