You said so yourself, it's a different mission. I'm sure both groups have extremely tough physical requirements, and the training is grueling. The only thing about Force Recon training that I would find might be difficult for a Ranger to go through is the water/SCUBA training, it's very similar to what the SEALS go through, so for someone who isn't used to water it might be pretty tough. On the other hand, from what I understand Rangers really stress extremely long endurance marches, much like the SAS, and that might not be something Force Recon Marines are geared towards. Of course, as a Marine myself, I like to think the Marines are the best at EVERYTHING, but to be realistic I'm sure anyone involved in one of our special operations commands, whether it be Force Recon, Rangers, SEALS, Para-Rescue, Delta, etc. is extremely dedicated and willing to put everything on the line in order to make it into that elite unit.