How long is the game total? Do you think I'll have any complaints using an iPhone 5s? I am upgrading to the 6+ as soon as my Verizon has them in stock.
And since you seem to enjoy the same kind of games I do... Have you tried the GBA emulator for the iPhone? I just want to throw this out of left field to a fellow Fire Emblem fan... The Last Promise is a complete rom hack that very well could be one of the greatest games I've ever played and I highly recommend you at least Google it.
I think the game can take 8-10 hours on the extremely fast side if you are playing easy or know what you're doing and skip dialogue and any extra battles you can (there's a risk/reward mechanic where after fighting a battle, you can attempt another right away for possible items or extra exp to level up. I always chose to do them since exp can be scarce, but if you didn't play the first battle well, the second one could be really tough and you might die since you start with whatever health and units you have left and not fresh).
I personally took 20+ hours, min/maxing my characters efficiently which you are used to as a FE fan. Spreading exp is easier, since kills are all that counts, not what kind of baddie was killed. And you might take longer should you lose a key character or decide to invest in one for their different skill sets.
I think the 5s had a newer chip than my itouch 5th gen, so it might help. Dialogue can be a little tiny and if you can, I recommend on a laptop or iPad for a better experience. But if you are used to srpging on a mobile, you would probably still be ok.
And I have played TLP and enjoyed it as well. The sole creator did a great job with the animations and music. You are probably also looking forward to the Immortal Sword project as well? It's been in the making for a long time but the main dev makes awesome sprite animations. And then don't get me started on playing Japanese FE hacks with really good sprite work!