They seem really similar because they are the same. ProTrek is the EU/JP name, Pathfinder the US branding, but they are the same thing. Sea-Pathfinder is not the same as Pathfinder and it is used world-wide.
Originally Posted by DavidSeattleWA
The EU/JP ProTreks have model numbers starting with PRG (non-atomic, but sometimes solar) or PRW (solar+atomic) while the US Pahfinders use PAG and PAW. As far as I know, the prefix of the model number is the only difference.
As far as functions, there are only a few series (models on the same row are the same given or taken the solar/atomic features):
The letters after the series' number only specify materials and colors (Y = all back w/ reverse display; T = titanium bracelet; B = black resin; J = Japan-only; GC, GB... = special colors).
Last, Casio also makes ProTrek-like watches which are not ProTreks but have some A/C/B/T functionality nonetheless, like the SGW-100 and GW-9200 (G-Shock Riseman).