In my personal opinion, you shouldn't buy either of these iron sets right now. Although either one would likely be a slight increase in quality over your starter irons, I feel it would be a waste of your credits. Right now, I think you should concentrate solely on gaining daily XP. Increasing your level takes precedent over lowering your score.
My recommendation is to wait until -- at the very minimum -- level 59, and purchase the Ping G20s. They have a very favorable meter speed, but more importantly, they have HIGH trajectory. A high arching trajectory is crucial to limiting 'roll out' so you can get close to that pin and give yourself makable Birdie putts.
Not only are the G20's better than the RocketBallz or R11s, they will easily last you until you hit level 75+, when the Taylor Made SpeedBladez become available, or you could wait until level 79 and purchase the Taylor Made RocketBladez. This isn't necessarily required, however - I've heard reports of folks using the G20s all the way up to the Legend tier.