Definitely get the sim first! Here are my reasons...

1) You can see how you like RC planes and/or helis before you make a larger investment to actually buy one (and buy all of the support equipment necessary for it)!
2) You can get much more experience for your time and money... crashes cost nothing and your plane is rebuilt instantly by hitting the space bar!
3) Once you get so you can consistantly take-off and land in the sim, you will probably be able to solo after only a few flights/sessions with an instructor when you get your real plane (I did).

Flying a real plane will be different, but most of the things newbies need to learn, you can learn very effectively using the simulator. Just keeping perspective on the controls when the plane is flying: away from you, towards you, inverted & away from you , inverted & towards you, overhead, etc. will take some time, and the sim will teach you this quite well. Learning how to make turns, do loops, rolls, snap rolls, etc. will also be easily learned in the sim. Turn some wind and gusts on in the sim for a little more realism once you've mastered things without it.