depends on application. oem wishbone bushes can't be bought on their own, you need to buy the complete wishbone from renault for £75 (x2) (but comes with ball joint too). you could try getting aftermarket wishbone bushes on ebay but this is likely a false economy. powerflex wishbones bushes need welding in.
powerflex bushes = less flex, more stress on components, harsher ride, last longer, i presume a more focused drive
oem bushes = more flex, less stress on components, more forgiving ride, won't last as long, i presume a more comfortable ride
unless you are planning on keeping the car for a good few years i doubt you would ever have to change them again, even if you want oem.
i had this dilemma a while ago. i went with oem arb bushes as they were cheaper than powerflex and they would be easier (i presume) to fit.
for wishbone bushes, i plan to go powerflex, but only because you can't buy the bushes separately from renault.