but what happens if I get a JAR CPL.
You can exercise the privileges of it in aircraft registered in other JAA States, otherwise it won't make the slightest difference.
You can have both licences, a good idea if you want to retain your IMC privileges without having to do a renewal, but by about 2012 Mr EASA will take it all away and you'll have to have a JAA licence anyway.
Unless you need a JAA Licence stick with it.