[Samsung Galaxy Tab S3 Guides, News, & Discussion] Ambits:
I am one of the people who went the opposite way of the OP. I only had the iPad Pro for ~4 hours before returning it. I certainly did love how responsive it was (definitely a combination of the powerful processor, well-tuned OS, and 120Hz display). I also can say without a doubt that applications for Apple devices in general are more polished than for other systems. Plus I know they are well supported for years with OS updates. That said, the iPad Pro fell short in places that really bothered me.

First is that there's too much of an expectation that you're all-in on Apple. For example, LastPass autofill only works on a handful of applications that explicitly add in support for it. On Android devices, the accessibility API and alternate keyboards give ways to handle the autofill problem. It's perfect (heck, even the Android 8 autofill feature is far from perfect) but it's better than having to swap apps and use the clipboard to fill the fields. If I was tying myself to Apple services, the autofill pains would be avoided because Apple Keychain is given special access to let it do global autofill.

Second was the lack of customizability (which I guess the first is a subset of...). Specifically I love being able to swap out the launcher without rooting or installing a custom ROM. For some people the launcher might me a small thing, but it's where the main button on both devices brings you when pressed, which makes it a big part of the experience to me. The first thing I did when setting up my S3 was install Nova Launcher and customize the settings to my liking.

Third point that bothered me was the Lightning connector. Apple has gone all-in on USB-C on their laptops; why do their mobile devices still use a proprietary connector? It wasn't a deal-breaker on its own, but I still find it dumb that they haven't killed Lightning off yet.

Finally, and most importantly, the Apple Pencil sucks. I got the tablet to be able to do sketching on a lightweight device and the faults of the Pencil were difficult to ignore:

I totally understand the complaints against the Tab S3 being made of all glass and think it's stupid, but I think it's stupider to make a slick stylus. It want my drawing implement to have some grip to it.

The tip of the Pencil is too hard. I could feel that I was dragging it on glass and it amplified the difference in feel of moving over a spot with a tiny speck of dust or a little extra finger oil. The tip that comes installed on the S3's stylus has a softness that feels much nicer.

The lack of any way to switch to an eraser without tapping to change tools was another pain point. The S3's stylus has a button that is perfect for quickly changing to an eraser, but being able to turn the Pencil around to do the same would have been a great feature if they really have something against buttons.

There's no indicator when hovering with the Pencil. I know the size of my line is going to change with pressure when using certain tools in a drawing app, but it's nice to be able to see the bounds before pressing the stylus against the screen.

And of course what would a complaint about the Pencil be without mentioning how it charges? At least there's an adapter to not have to plug it into the bottom of the iPad where it would be way too easy to break. But still, I have to take the cap off the connector and I can say with certainty it would not have taken long for that to get lost.

My overall opinion on the Tab S3 vs the iPad Pro is that they both kind of suck but, for my desires, the S3 sucks less. I can deal with the lower battery life, especially with fast charging. And the general slowness/lagginess isn't as perceptible when drawing as it is when scrolling lists or watching UI transitions.

With all that said, an Apple Pencil 2 that fixes the large list of Pencil faults would have me at least reconsidering the iPad Pro. However I'd still prefer a better Android tablet unless Apple opens their system up more (ha).