I dont think it's so much that the Cub is "hard" to fly, it is the fact that the Cub is more of a scale aircraft, and scale aircraft fly differently than trainers and other sport type taildragger aircraft. Which Cub do you have? Most I have seen have a fairly scale wing, in that it is thinner, with more of the characteristics of the real full scale Cub. Your trainer, like other trainers, has self correcting tendencies that will help it right itself in the event you lose control. The large amount of dihedral in the wings of most trainers makes them inherently stable. The Cub is not going to have that same ability, and it's going to go for the most part exactly where you point it. If you've flown 5 gallons of fuel through your trainer, I'd say you've got a good bit of experience. I've heard instructors say "if you can consistently fly a complete circuit of the field several times inverted, without loss of control or orientation, then you are ready to move to another plane". I dont agree with that. I dont think that a person's skill level can be judged by how well they perform one maneuver, it has to be a combination of all their skills. Someone that is watching you and helping you can tell you "yes you are ready" or "no, not yet", but in the end YOU are the only one that can make the final determination as to whether you think you are ready to move on or not. Truly look at your flying skills and see how you feel about them. Are your landings consistently good? Do you maintain your orientation well in the air or do you have momentary lapses where you have to think about what to do next? All of that has to go into your decision. I dont agree with ANYONE, instructor, friend, whatever, telling a pilot that they CAN'T do something, and then not giving them any explanation as to why not. If you tell someone they cant do something, and then cant explain to them "why", you have no business advising them in the first place. I would say, if YOU honestly think you are ready for the Cub, get an experienced pilot on a buddy box with you for the first few flights. That way, you will be able to learn the aircraft's handling qualities without fear of destroying your plane in the process.