SP can hit 35-40km/l ??? 
No matter how slow I ride, my SP could never hit 30km/l, let alone
35km/l.
I think there's something wrong with my bike, or my riding style
Or most probably the person who told you that was exaggerating
Back to the original question, I owned both RXZ and SP before. Both
were in stock condition. My RXZ averaged 28km/l. My SP averaged
21km/l. Some people will claim higher consumption, some people will
claim lower consumption. It all depends on their bikes conditions
and their riding style.
So from consumption alone, keeping an SP is more expensive. Whether
you will feel the pinch or not, it's relative to your spending
power. If you're still a student, difference of 20 bucks per month
might be significant. If you're already working and earning a
comfortable salary, difference of 50 bucks per month might not
matter. I personally find maintenance and spare parts costs don't
differ that greatly between those two, and both are reliable
bikes.
Performance wise, of course SP kicks RXZ's ass. And don't listen to
people who say Mr ABC's RXZ can smoke SP after changing this and
that. We're talking about RXZ and SP in general, don't use a single
bike as benchmark. There's no replacement for displacement.
Appearance wise, this is subjective. But in my eyes SP wins hands
down.
Comfort wise, RXZ provides better riding posture for commuting, and
it's more pillion-friendly. Riding SP at slow speed is a pain in
the neck (and shoulders), literally.
