Snell is the racing standard (if you go to a track day, they'll ask you for a Snell rated helmet) and ECE is the European standard (their version of dot i believe).
Both are good. Generally speaking Snell helmets are heavier and more rigid and ECE tend to be lighter. Either way, if your lid has either of them you have a quality helmet. Keep in mind some helmets have only dot but are still excellent, Shoei for example have a helmet with a flip sunshade visor based on their racing helmets, the flippy thing doesn't allow it to pass Snell but it's still a very safe helmet.
Dot is a garbage standard (check the video) but that's what is required in North America. Pretty much any piece of foam with a plastic cover can pass dot. So a cheap dot helmet isn't gonna do you much good. You can find a good full face in the 150-200 and an excellent one in the 400-500. A $35 helmet will keep the police at bay but don't expect it to do anything for you if your head hits something hard.