I would choose iPad because I use it a lot more, but on my second-to-last upgrade I chose my iPhone and I couldn’t have been luckier. I went from a perfectly working iPhone 6s on iOS 9, to an iPhone Xʀ (on iOS 12). I had, back then, a 9.7-inch iPad Pro on iOS 9. I chose to upgrade my iPhone, and shortly after upgrading, Apple forced all A9 (and A9X) devices on iOS 9 to update due to a deactivation bug for that specific combo (A9 on iOS 9).
Unluckily, and shortly after I got the Xʀ, both of my main devices before upgrading were there, and they were both forced out of iOS 9. The 6s was forced into iOS 13 (which obliterated battery life. I gave it to a family member, but I tried it, and it went from 8-9 hours of SOT to around 3. Unusable), and the 9.7-inch iPad Pro was forced into iOS 12 (which severely impacted it too, but a bit more moderately. It strained (still does…) to get to 9-10 hours with very light use (got 14.5 on iOS 9), but it wasn’t as bad as the iPhone. Performance also worsened a little, but nothing too bad. I had keyboard hangups on an iPad for the first time ever. Not too bad and not too frequent, but it was perfect on iOS 9. After using it for 3 years on iOS 9, I noticed absolutely everything. Had I chosen to upgrade my iPad, I would’ve had to stay with the 6s for quite a while, and not only was battery life abhorrent, the LCD broke too, and it had short lines at the top of the display. Usable, but not good.
Now it’s all good. My iPhone Xʀ is flawless on iOS 12, it lasts like 16 hours of SOT with light use, it’s insane (performance is perfect, too!), and in spite of the fact that I used the 9.7-inch iPad Pro for several years whilst being severely affected by iOS 12 like I said, I got the iPad Air 5 with an M1 and couldn’t be happier with that combo.
I don‘t care if my device is old. I don’t care if it’s 7 models behind. All I care about is how it performs and how battery life is. As long as that is good, I’m happy. I think I use my iPhone 6s on iOS 10 more than my Xʀ. It’s one of my favourite Apple devices ever (alongside my new iPad. It was long overdue).