I recently switched from an (SKT) Samsung Galaxy S4 LTE-A to an iPhone 6 Plus. Prior to the S4 LTE-A, I used an iPhone 4S. I ABSOLUTELY LOVED my S4 LTE-A, the LTE was super fast, and battery life was great until after a year of use. Then, it just didn't hold a charge that long. I LOVE my current iPhone 6 Plus as well. The battery life is unlike any other device I have ever used. I can get through a full day of heavy use and still have 10-20% of battery left. There are some things that I miss from the S4; however, the iPhone 6 Plus is a welcome change. If you are thinking about changing it up and going to iOS, I say go for it. I am glad that I did. I have been using Apple products for years, Macs, iPads, iPods, and iPhones. I welcomed the change to Android, it was a great experience. I think that going to iOS is a nice way to keep things fresh. I would have enjoyed a Galaxy Note 4, however, I think it wouldn't have been too much of a change to excite me since the interface of the S4 and Note 4 is very similar.
If you are debating between an iPhone 6 or 6 Plus, get the Plus. One reason is better battery life. The other is the screen. Right before I went to the 6 Plus, I used a 5S for 2 months. It was VERY difficult to go from a 5 in S4 to a 4in 5S. Yes the 6 is a bigger 4.7 inches, but it is very difficult to revert to a smaller screen. Get the 5.5 in 6 Plus.