I have both, and use the iPad much more than the phone - I prefer the larger screen size & use Safari for most internet stuff rather than apps. I always feel when using apps for internet as if I'm not getting the whole picture, it feels like peering through a letterbox.
Also I don't see the point in switching from one app to another when I'm moving between say Mumsnet, FB, Amazon & a news site - I just use the bookmarks at the top of the page.
I also like being able to have two apps open with a split screen, so I can take notes from a web page using Safari & the Notes app, or have my password manager open next to the browser.
I play games, do my banking, check emails, watch tv & YouTube, read books, do jigsaws, look at maps and my photos on the iPad - all look better on the larger screen.
My phone I use mostly for taking photos, music in the car, texting, WhatsApp, (as there isn't a really satisfactory third party app for that on the iPad) to update my calories on My Fitness Pal, as it's easier to use for barcodes than the iPad, and to sync with my Apple Watch. And as a phone, of course.
I use my watch for fitness stuff, checking emails, notifications, receiving & replying to text & WhatsApp, answering the phone & contactless payments
I like the fact that everything syncs together, my photos I take on the phone transfer automatically to the iPad, I can check the details of something I want to buy on the iPad and then open the page on the phone in the shop and lots more. Today, for example I was making a shopping list for my DD, who does our shopping with hers every week. I went round the kitchen listing stuff I needed in Notes on the phone, then opened the Note on the iPad to edit it into the right order for DD to go round Tesco which is much easier on the larger screen.