there is nothing such thing as better life span or a card it all depends on the brands and pure luck ........
the gtx 760 is better than r9 270x about 6-10 fps better but also costlier and costs as same as r9 280 which is slightly better than the gtx 760.....
if u are deep into detailing then go for the nvidia it have good physx and will give u good effects like car busting throwing a grenade effects better than amd but u have to really have good eye for that....i hope it helps any more question feel free to ask.....
in preformance they are nearly the same as for nvidia having a slightly higher price tag.
now when choosing a gpu you will want to consider what games you are going to be playing on it.
amd might be cheaper but usualy consumes slightly more power than nvidias competitors in most cases.
there are features that nvidia has like gsynk and adaptive vsynk that amd lacks although amd is finding alternative with freesynk and amd mantle.some games that use physics play better on nvidia cards and in order to have dynamic fur or phisics you need to have a nvidia card.
some technologies of one brand are just incompatible on some games or technologies.when you buy it you will be stuck with it so take time.