They all do pretty much the same job. First you need to work out the BTU requirements for your room (google radiator size calculator).
Basically for standard panel radiators, you can have single convector, panel & convector and double convector depending on the size and output. Of these you can have the roundtop (the most expensive) such as the Myson Premier HE, the standard type, and the compact (with edgings and top grille such as The Myson Compact).
Then there are the designer ones such as the flat panels and column ones.
If you are replacing an existing radiator, you are best to go with the same length as what you are taking out to avoid messing about with the pipework.