I've had an Apple Watch for more than two years (still wearing my Series 0 Sport), and I think an Apple Watch is very much worth having for me. It's not as useful or as desirable for everyone though, so I think it depends on how much you think you'd get out of it. That can be a tough question to answer if you haven't worn one. If you use notifications a lot, and/or you want a fitness tracker, and/or you compulsively check the weather or calendar, the Apple Watch is fantastic.
I will say that I do not use my iPhone as much since getting the Apple Watch. My iPhone stays on my desk, in my bag, or on a speaker dock much more than it used to. I use my Apple Watch for quick interactions and my iPad for longer interactions. When I bought my iPhone 6 (shortly before I got an Apple Watch), I did not want a 6+ because I did not want to have to carry and hold such a large device when I already had an iPad. If I did not have an iPad, I would much rather have a Plus model or the iPhone X for their larger displays.
In addition to thinking about what you might get out of the Apple Watch, think about how you use your iPhone. If you are really into iPhone photography and video, then the 8+ or the X will give you much more camera than an SE or a non-plus 7 or 8. If you don't have an iPad, then the 8+ will be more comfortable to read.
If you really want both an 8+ and an Apple Watch, but buying them together is out of budget: Ask yourself what you would rather have first. If you are getting by with your current phone, get an Apple Watch now and a new iPhone in a year (or less). If you want a new iPhone now, get the 8+ and either wait on the watch or consider buying a used and/or older model as long as you don't need LTE. My Series 0 is still humming along, and I bet there will be some great holiday deals on the Series 1 and Series 2 models left in stock.
Sean