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would my personal preference outweigh any differences in objective things like quality and performance?
Personal preference is the only reason to choose between Shimano, SRAM and Campag. Whether it's worth the expense is something only you can decide. Since hood shape and shifting ergonomics probably make more difference to the user experience than frame choice, wouldn't it be better to pick a bike which has SRAM as original equipment?
The bike I'm about to buy has 11 speed 105. I don't really like Shimano (possibly because I'm used to SRAM). Is it stupid to swap 105 for Rival, or would my personal preference outweigh any differences in objective things like quality and performance?
I'm conscious that if I start trying to get used to it once I've bought the bike, the 105 stuff I'd be looking to sell would have lost value.