Hey there fellow old timmer!
I was in the exact same boat as you. I've been a iPhone and Mac user for years now. I did own a Moto Droid 1 for about a year and a half before switching back to iPhone (had iPhone prior) at back then felt like Android was to much of an unfinished product for my taste.
But I too was really feeling like I couldn't justify sticking with iPhone any longer with so many awesome phones on the market. I pre-ordered and received a day before the "official" launch two Samsung Galaxy S4's, one for myself and one for my wife. I switched carriers in the process as well from Verizon to Sprint as I couldn't upgrade my device and get it subsidized and keep my unlimited data. Figured why fight to keep a carrier and my unlimited data AND pay more than all the other carriers in the process? No regrets on any of it! Of course it's very different and takes a bit of time to retrain your brain on when using Android. But I can say that Android 4.2.2 is a big improvement over whatever the Moto Droid 1 was running.
The S4's AMOLED 5" 1080P screen is amazing. So freaking crisp and bright. Can't get over how amazing the screen alone is.
The phone is so light and slim that going from my 4s to the S4 (just realized those are just the reverse of each other!) I don't even notice the phone in my pocket most of the time. I was concerned the 5" screen might be an issue in the dress slacks I wear for work but it's been a nonissue.
I never felt like the 4s was a slouch or anything but I realized it was much slower at doing some every day things. The biggest thing was when I was neffing and killing time on the 4s, I would almost never bother trying to load up a .gif as it'd take forever and sometimes crash Safari. This wasn't due to the slower 3G data either, was an issue when on wifi as well. On my S4, .gifs load just as fast as if I was opening them up on my Mac/PC. Night and day difference for such a little but repeat thing.
In fact, everything is amazing response on the S4, I've yet to throw something at it and it show any signs of strain or inability to keep up. It's a powerful phone.
I was debating between the S4 and the HTC One for some time. I read the reviews of every tech site I could find. For a point I was leaning towards the HTC One but a few things swayed me over to Samsung.
One big (for me) reason I went with the S4 is the near iPhone levels of aftermarket products and accessories for it. No other phones other than the Galaxy line and the iPHones have this level of available products for them. This (to me) matters as I don't want to have a very limited amount of choices for cases, docks, batteries, etc.. that causes everything to cost more money as a result.
I've experienced that in the past and it sucks paying $50 for a case that had I gone with a different choice I could of gotten something just like it for $5.
The second big reason, one of the selling points to me about Android that I've missed out on as an option with iPhones....User accessible battery and Micro-SD.
I bought my 16GB S4 for $150 (with new service at Sprint) without worry about it being 16GB as I already have a ton of Micro-SD cards. A couple 64GB cards, a couple 32GB cards all Class-10.
That's really the biggest reasons I chose the S4 over the One.
Everyone likes to put a lot of focus on the S4 having a plastic back on it, but I've always put a case on my Smartphones. They just cost to damn much to not protect IMO.
I don't use the built in speaks on phones often so while I do thing the front facing speakers on the HTC One make great sense and make me wonder why something so simple has taken so long, its in the end cool yet not very useful feature to me.
Also, maybe it's being silly but I feel more confident in Samsung as a company. HTC is a floundering company that hasn't turned a profit in a long time. While I hope they recover from the One and keep the successes going this point forward, if they were to go belly up, where are you going to get your OS updates from? What about warranty issues? Will they let things like updates and customer service drop as a way to compensate for their shaky business? Again, just me probably but something that I feel at least isn't at all a factor with Samsung as they aren't going anywhere in the foreseeable future.
Aside from the Android phones, one BIG concern/pain point I had with leaving iOS was that I had SO MUCH sunk into the iOS app store and the thought of not being able to have access to the literally thousands of bought or downloaded in the past apps wasn't something I was crazy about giving up.
While yes I have repurchased a very few select apps that I really wanted on Android, it's really not been all that many. Reality is (and became clear after the fact) that yes, I have an insane amount of apps in the Apple echo-system that I'd never have to repay for, but I only used very few apps and of the those a even smaller amount were paid for apps.
The only thing I cannot use with Android that I could with iOS is bluetooth syncing of my Nike Fuelband since Nike has decided to kill off their plans to create a Android app. Not a huge deal since I can still sync it to my Mac but still something I do wish was possible and I'll probably switch to the Fitbit Flex soon since it works with both iOS and Android.
I was concerned about my wife making the switch more than myself since she's not as tech savvy but am impressed with how quickly she's adapted.
One last comment on this much longer than intended post...
I sold my iPhone 4s's and I hadn't touched them since getting the S4 but needed to get them factory restored and cleaned up to sell and when I picked it up and turned it on I literally laughed at how comically small and toy like the 4s felt in my hand. My wife and I both have said that we'd never be able to go back to a 3.5" screen smartphone after this.
I'm happy to try and answer any questions you might have on the topic.