Actually I think it's the bottom bracket that's flexing, not the cranks ... or maybe a combination thereof.
I found the new Shimano XT M760 cranks/bb combo set to be extremely stiff ... if there's any flex at all, it's the frame (it's an old Trek 7000 mtb frame). I also weigh about 135 lbs so I'm not that hard on equipment.
Some flex is normal. If it's really bugging you, I'd go for the new set.
I wouldn't use the XTR cranks if you have concerns about longetivity ... the chainrings are all aluminum (for weight savings), so they wear faster.