I will tell you that no matter which one you choose, you won't be dissappointed. I'm not huge into supps like many people here so I pick and choose very carefully what I'm going to take. Besides the basic protein, multi, fish, BCAA, etc. they only 2 I've done are Leviathan and RPM. I'll be honest on what I felt (same diet during both)
Leviathan:
Smoother energy, calmer more consistent friendlier energy
Felt somewhat hot a lot of the time throughout the day
Helped me lean out and retained muscle gains
I did not lose any weight while on it but I feel that I did lean out, probably gains in muscle what I lost in fat (3-4lbs in 2 months I'd say)
RPM:
More aggressive energy, sometime almost angry irritated engery
Felt very hot about 1-3 hours after taking it but not so much during the rest of hte day - sweat like crazy during the workout.
I feel very sore after lifting now b/c I really push it when lifting.
I'm 3 weeks into it so it's hard to say the final result but so far have made some strength gains in the gym but body weight has not really changed. Maybe 1 lb in 3 weeks.
Bottom Line: You can't go wrong with either one. If I had to pick just one right now I'd go with RPM b/c I currently like the aggressive nature of it, but I would not hesitate to go back to Levithan if I need to cut some fat again or am at a point where I'd rather have the calm aggression (oxymoron?) instead of the rage.