If i was 6 - 15 and looking to join a club I'd definitely join Chelsea. Mixing with high quality players from around the world and getting coaching which is second to none would be a huge draw for me. However, if I was above 15 I think Brighton may be a bigger draw for me. Once you hit that age game time starts to matter more than training techniques and at Chelsea you'd have to be something special to get regular game time for the development squads, whereas at Brighton there's actually a good chance of progressing to the first team if you perform well in the DS. How many players have come from the DS from Chelsea compared to Brighton in the last few years?
Having some experience of both camps I would not advise any kid to join Chelsea.