The most important and biggest difference is that X10 Berlin is real and BRT is fictitious in terms of lines and streets. Of course BRT has new real houses but the rest is not really real and let's face it BRT C2 is unrideable without mods.
A positive aspect of both add-ons is the double decker, which is very real except for the matrix and some sounds.
My recommendation is that those who like realism should buy X10 Berlin first and those who absolutely want a C2 (of which there are not enough) and can cope with many bugs and do not value realism can buy BRT.
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