Never heard of picking up rail tickets through a CU, so no idea how cheap/expensive they might be.
Bus will be the cheapest option, but the journey is a long one.
It is possible to pick up flights cheaper than the standard rail ticket (~€45 last time I checked on flights vs. €59 rail), but as always depends on when you're travelling.
Given the location of the airports and where you're travelling to, it might be faster (and cheaper) to travel by rail (both train stations with central locations, both airports a fair journey away from city centre).