When it comes to SRAM X.O. which makes more diff
If all cables and housing are equally clean/new/unkinked, which makes more of a noticeable diff in shifting; upgrading to X.O triggers or the X.O rear derailleur. I thought one was just "eh, not that much" better than X.9, but one was "wow, that is definitely worth eating Ramen for a week."
shifters always tell the der what to do..... kind of like the brains of the system. so I would opt for the shifting upgrade. then the next time you rip the der off the hanger you can justify the money spent on the shifting units and/or then upgrade to the XO der
and I always run high end shifters and assume they will last longer than my cheapy ders b/c they are less likely to be removed from my bike during a rag doll incident.
just my $0.02