since you have so many apple devices, i think it would be easier for you buy an iPhone, just so you have icloud and all that stuff. I know the 6 and the 6+ are out, but budget wise, the 5s may just be the better choice if you choose iPhone. I'm not a big fan of the iPHone 6, and now that the iPhone 6s or the iPhone 7, i'm scared to see how far back apple has traveled. Sorry, this is just me right now, but apple is going the wrong way as far as technology. thinner, is not better, a huge bending sheet of metal, is not better, more random crap, is not better... That's why i would suggest the 5s, cause it's cheaper, looks nicer, i believe has a better battery, and still works with along with all your other apple device.
But if you are on a budget, i would go with android. Right now, the best buy is either a lg g4 or a oneplus one. the lg g4 is going for sale, at my carrier at least, for around $400. That's really good considering that it has the best screen, running at 2k, the best camera. yup, better than samsung, faster than an iPhone. and it's just a nice phone. it still has the removable back. Only issue is, the battery life is kinda dull. Well, then again, what modern smartphone doesn't have a dull battery... The other best buy is the oneplus one. It is still good. and for $299, you can buy the 64 gb version, so that's a really good deal. Since every smartphone is not perfect, the flaw is the touchscreen. i don't know how well the issue has been addressed, and i don't know if there's a fix for it yet, but there's a touch screen issue with the oneplus one.
These are, just my opinions and views.