A big difference, a bit like the focal Access range compared to the focal polyglass range.
theres always better, you pay more, you get more. unfortunatly in rainbows you can keep paying more until you get to there £6500 (yes thats the right amount of zeros lol) 2 way component set!
i wouldnt get the slx230 non deluxes if you dont mind spending the extra and getting the deluxes. the 'crossover' on the slx230's are very close to the design of a 'cap' and not a proper crossover. The sound quality is very noticable between the deluxes & non deluxes whether your running them off an amp or not.
There are many places under the dash & behind the door panel to hide the crossover so that shouldnt be a negative factor in buying these speakers mate
Specs are similar all the way up in the rainbows power & frequency wise, but the designs and materials used differ as you go up. also the tweeter changes between the models, but off the top of my head i cant remember if the slc's use the same silk dome style tweeter as the slx's... i know they changed from ferrer fluid to air cooled at one stage, but cant remember if its between these two or if its when they go to the power & germanium range :S sorry i couldnt be of anymore help...