I did not purchase S4 because of screen size but primarily for the ECG capability and so far I am impatiently waiting....what was your reason for buying S4
My only concern for you long term is OS capabilities - not sure how many more iOS updates the SE will get and you typically have to have your iPhone updated to the latest iOS before updating your Apple Watch. Just keep that in mind (unless you already knew it) before bashing the devices a year from now.It was a series of circumstances that fell into place in order that was why I bought it.
TL;DR version - I needed a new running watch, but wanted a smartwatch, and this does both.
I'm a runner and I've been using a Garmin watch ever since I was recommended it years ago by a fellow runner. My first Garmin was great as a sports watch (non-existent as a smartwatch), but the strap broke at a position that made it impossible to replace without great expense.
My second Garmin watch was the Garmin vivoactive (easy to replace the straps!). Not quite as good as a sports watch (good enough for me) with some smartwatch leanings (bluetooth, phone notifications, control music playback, etc - no HRM).
My Vivoactive is now over three years old and I was looking to replace it. Initially, I leaned towards the latest Garmin Vivoactive 3 because it had HRM, plus Garmin Pay (I even set up a bank account that could do Garmin Pay in prep for the watch). I delayed buying it because a slightly newer Garmin Vivoactive 3 Music was being released (same thing but can play music direct from the watch).
I saved up around £279, waited for the new Garmin, then waited a bit longer to see if it would reduce in price in Amazon sales (once or twice it did, and I very nearly bought it).
Just as I was ready to buy the new Garmin, I stumbled across an affordable deal for an iPhone SE (my Android phone had suffered a broken screen, and my "running phone" was a cripplingly slow Windows phone - the SE replaced both).
Then it occurred to me that, now owning my first ever iPhone, I could take a look at the Apple Watch - something I couldn't do before due to not having an iPhone. I knew it was more smartwatch than sports watch, but a fellow runner had an older version and he said it was up to the job.
And so I looked at the Apple Watch. Its smartwatch capabilities drew me in but, just before making a final decision, I heard that a new "series 4" was likely to be announced. So I held off buying once again and used the delay to build up my watch budget (I figured I'd need £100 more than the £279 of the new Garmin - turns out I needed £150 more!).
After all the pieces fell into place, I got my Apple Watch series 4 Nike+ and have now taken it on a couple of runs. So far, I don't regret any of the above. (Would have been nice if it was cheaper, though! )
That is my only concern in the back of my mind.My only concern for you long term is OS capabilities - not sure how many more iOS updates the SE will get and you typically have to have your iPhone updated to the latest iOS before updating your Apple Watch. Just keep that in mind (unless you already knew it) before bashing the devices a year from now.