you said you believe Leto's was more powerful, I would agree that he would seem to have more control, but can that be interpreted as power?
OM to me, depends on the user, the more you use it, practice with it, the better you get.
In heretics and chapterhouse the Mother Superior and a few higher ups are seen using their OM more then a lower RM would. (Odrade, Taraza, Sheeana)
Although not Frank's books, Anirul uses OM extensively much more so then other RM's. The biggest risk was getting lost among the voices.
Here is where I think Leto was stronger, not more powerful, but strong enough to go into the voices and return safely. And this strength came from hundreds of hundreds of hundreds of years of living with them.
Superman was powerful, Leto has strength :-) (NT) -- The Cannabist, 19:23:45 07/09/03 Wed