As a relative new OCS player (I got introduced to the system this year) I would say RE because it has a low counter mix and comes with the OCS Starter Guide/Walkthrough. RE does a good job of giving you a taste of a lot of the mechanics of OCS in a digestible format. I have not played Sicily II yet as Baltic Gap is currently on the table; however compared to Sicily I (Triumph and Folly) its more streamlined and a smaller scale. As mentioned by Tankboy Tunisia II is a very smooth playing OCS title, which I enjoyed very much but I would say start with RE first and then go crazy if you enjoy the system.
Some very good advice. I haven't played Sicily II either but just looking at the airdrops and amphibious assault... those things will be of no concern in RE and will make it much easier to start playing the game
Minor thing about Sicily II. You still need to print to standard rules and combat tables. A bit annoying when your printer decides to stop working like mine did a few weeks ago