I was in a similar situation. I wanted the 6+ since it was rumored, but when I saw it at launch at the Apple Store, I said, "way too big". So I opted (in my mind) for the 6, until I was ready to make the purchase back in Dec. 2014. Someone here at MacRumors mentioned the extended holiday return policy in a post. I went into Apple, got the 6+ on a whim and I never even gave the 6 a consideration from that point on. The best part was, 2 days prior to my final return date, the sim-free was released. I exchange my 6+ T-Mobile for the Sim-Free Version.
Go to Apple and mention to them your dissatisfaction with the 6. It doesn't hurt to try. If no success, sell it and get the 6+ or wait for the 6+S.