To be honest Angus, it's not only the speed controller spec you should be interested in, it's the quality of the product and the backup service / warranty issues need to be checked.
Me, I'm a cheapskate, so my decision making process usually goes...
1. What spec. do I need?
(Max volts, max Current Amps?)
2. Buy ESC with at least minimum voltage and 2 x max estimated current.
( Votage needs to be closely related to selected battery voltage for best control, eg, 6, 7.2. 12, 14.4v etc.)
( For 'reputable' ESC manufacturers, Amps are pretty much a conservative estimate, ie 20A ESC will usually happily handle 25A.)
( For "Chinese" ESC ( ie. non labelled, no website, cheap imports), estimated Amps x 2, x 4 or even x 10! .... "Chinese" Amps seem to conform to a different interpretation of Ohms law... )
3. What 'sort' of ESC do I want?
( For scale ie good low end control, linear speed range. Speed race, ie. handle a lots of power ... and abuse!)
4. What feature are required?
( 'Normal' or Brushless - obviously. But things like BEC, reversing, brake, "self setting", programmable, etc.)
5. What physical size?
( Will it fit? )
6. Will water cooling be required?
( Usually applied to power boats. )
7. What connectors does it come with?
( Am I happy changing, soldering different connectors as required? )
8. What manufacture has "best" reputation? What do others recommend on here?
( Buy one of these! )
9. What am I willing to spend?
10. Buy the cheapest ESC I can possible find on Fleabay. Burn it out. Rant at poor Chinaman that sold it to me. Get another one sent from China. Burn that out too. Swear and curse all Chinese take-aways as I wait for the the smoking boat will drift back in and hopeit reaches shore before it sinks. Throw boat to back of shed. Resurrect said boat from back of shed a year later, having conveniently forgotten where I got the unlabelled ESC from. Buy another one from Fleabay, get lucky and sail way past sunset on a warm Sunday evening wishing I had better low end control. Go back to step 10 and do that!