I'd say it's pretty much a case of you get what you pay for with this particular component. A cheap third party one will only set you back around £80-120, but it'll probably only last you a couple of years tops. A genuine Peugeot cat will set you back noticeably more (around £320 from a dealership last I checked), but provided the car is running correctly it will also last for considerably longer.
If you did go for a genuine part then it would probably be advisable to check the condition of the lambdas at the same time - if they're particularly old and/or not as responsive anymore, they could reduce the lifespan of any new cat fitted. No point forking out for a better unit if it's just going to get ruined quickly anyway.