Okay, I've written this in a bunch of other threads, but I'll add it here too.
Kitkat is cripplewear. Yes, you can, in some instances, run apps from the SD card, but there are many that you CAN'T. Additionally, VERY FEW apps will WRITE to the SD card, thus making it less than useful -- depending on how you use the card. If you want to use it as a ROM that you dump things onto from your computer, then sure, you're good to go. If you use it for document editing/picture editing/ encryption of ANYTHING, you're out of luck unless you root the phone.
Now, about rooting. That really depends on what carrier you're with. I can only speak to AT&T -- who has released a continual stream of "Security" updates that make it very difficult -- but not impossible -- to root the phone. Please note that I HAVE NOT TRIED THIS YET, but here is the thread on XDA. There's a lot of info in the thread and you should probably read it before attempting to root your phone. And again, as far as I know, this is ONLY for the AT&T phones.
[How-To] Root G900AUCU2ANG3 | AT&T Samsung Galaxy S 5 | XDA Forums