I'd say the RSL. However, depending on your swing speed, you may think differently. I'd advise you to try both rackets. What is your swing speed (fast, average, slow)? The stiffer RSL will suit quicker swing speeds but the flexier Yonex would be better for a player with a slower swing speed.
The Arc10 is awesome. I own one. It's stiffer than the Arc7 though; good swing speed is more important. I'd still recommend the Arcsaber 10 over the other 2 though. It's an awesome racket.
The RSL has the potential for a greater smash compare to the Arc7 because it is stiffer. But unless you have good technique and can swing very fast, you will find the smash weaker than the Arc7. Don't be mistaken, the Arc7 is no slouch also. However, if you are looking at the Arc Saber series, and unless you are very very defensive oriented, I recommend the Arc10 instead as the prices are very close and the Arc10 packs more punch.
Though the Arc10 is more demanding racquet than the Arc7, it is by no means a very difficult racquet to use. There are racquets out there that are difficult to use which I won't recommend to anyone unless they are strong players (like some of the forum guys here). I don't see Arc10 as belonging to this category. I think it is better to get something that has additional head room so that as your skills improve, you can utilize it more effectively.
Difficult ones are RSL X2 Gold, X2 AP, Yonex AT700, Mizuno TC700, MP100 and Ti10. I won't recommend the above to beginners for sure.
It is also because Arc10 is head heavier than the X2 AP, that's why I feel it is easier to generate more power.
