Like others have said, plan to pay $25 for SunShine. If you can S-OFF with the free tools, that's great (I was able to) but you just might have to pay. Consider that before jumping. S5 can be rooted for free with TowelRoot, then you can flash TWRP/CWM and be on your way.
The big thing about coming from any Samsung to anything else is you will automatically get better signal. Samsung goes to the dollar store for their radios/modems, and they buy refurbished to boot. (Not literally, but it seems like it.) I came from the S3 on a different carrier. Night. And. Day. I went from having to put my phone in the kitchen windowsill to make or receive calls. Now I can make or receive calls in bed. Part of that is Verizon's network, as opposed to the other one... but Samsung radio quality (or rather the lack thereof) is well documented. The S5 is newer, but the S3 was widely considered the best smartphone of all time for a good little while there.
To its credit, the S3 seemed to have better ROM compatibility. Except when it didn't. Most of the time it was just fine running AOSP-based firmware. Then it would act up. "Cannot connect to camera." Reboot. Get a few more pictures, then it happens again. Reboot. And it would always do it at birthday parties when I really needed a dependable camera. But for the most part it was golden. The One M8? Just doesn't feel right using anything but Sense. I don't use Blinkfeed and I think I'm about to replace their locker with WidgetLocker, and I wish I could replace their quick settings toggles (I really just want flashlight and screen off in there), but the ROM itself (well, CleanROM) is rock solid. It's no longer like, "My phone is okay, but _____," it's more like, "I have the best smartphone on the market."
Have you ever seen an ad for a smartphone, looked at yours and said, "Gee, I wish I had THAT phone?" Or maybe not an ad, maybe a spy show, something like 24? When the latest season of 24 started, I had my Galaxy S3, and they're up there rocking Galaxy S5, iPhone 5S, and I forget what else... I think it was a Windows Phone 8.1 model... and it's like, "Okay, my phone's 2 years old, of course the spies on 24 have better phones." But then I got my One M8, and it's like, "Wow... I have a better phone than Chloe O'Brian, or any of the CIA/Secret Service people... Nobody on this show has a better phone than I do." It's a good feeling, to see someone rocking an awesome phone on TV, and to be able to think, "Mine's better." Or maybe it's just me. And I'm usually not like that.
HTC One M8 / Verizon / CleanROM