I switch between the regular series watches and an Ultra on a regular basis. The ultra is a bit bulkier, but you get used to it. I do think the series watches are much better to sleep in terms of comfort.
Overall, I think the Ultra watches are just better built / better tech. Much better battery life, the flat screen looks attractive, and the bulkier look is just a style choice more than anything else. My first generation Ultra is still at 100% battery, and is one of my favorite Apple purchases over the past several years.
The thing is the Series watches are also good products, and can functionally do most of what an Apple Watch does
This one, there is no real right answer, just personal preference