You need to be clearer in your goal. If you want to work for an airline, there are quite a number of systems used for airline reservations. Travel agencies have the primary 4 GDS's (Sabre, Galileo, Worldspan, and Amadeus). Many airlines use other reservations systems (SITA, Navitaire, "home grown"), but many are also "hosted" in a particular GDS.

It is true that these systems are very much alike, so once you are trained on one, the concepts should be familiar and learning a new one should be relatively easy. In general, Amadeus would be the best background knowledge for working for a European carrier. Domestic US, it would not seem to make a great difference unless you had a specific airline you wanted to go for.