So WWDC happened since I created this post, which does have some influence on my thinking. Before I was on the fence about upgrading at all. I'm a big podcast listener though, so that addition in watchOS 5 sealed the deal on upgrading for me. I will, without a doubt, get far more use out of the series 4 with LTE, waterproofing, and podcasts than I do my series 0. I'm taking a cruise in January, so I would be very pleased if they somehow improve the waterproofing to 100m, but I'm not holding my breath on that one. Not sure I would trust a 50m waterproof watch doing something like SNUBA, even if I do only go down to 30-40ft.
My current thinking has me going back to why I bought the SS in the first place, and that is the thought that I don't want to worry about having to baby the watch. If I'm going to wear a watch I want to wear it every waking moment, no matter what I'm doing. The watch loses a lot of utility for me if I have to take it off when I'm doing more nitty-gritty things, like yard work or doing an oil change on my car. It's the same reason I've mostly stuck to the sport band, I don't want to have to worry about staining my leather or nylon bands when I do the dishes and whatnot.
I will say that my series 0 has served me well. It has been on my wrist at work, conferences, business trips, doing yard work, working on my cars, while cutting up trees with chain saws, packing and unpacking for a cross-country move, keeping track of complex medication and care routines for 3 senior dogs (who have all sadly passed away), commuting 70+ miles to and from work each day, the list goes on and on. Through all of this it is hard to boil down all of the benefits the watch has given me down into a tangible list, but it has done more for me in these last three years than any watch I've owned before ever has.
So in that sense perhaps it isn't so hard to justify spending the extra money for the stainless. It also helps that I'm getting a bit of a bonus from work that I wasn't counting on that will easily cover the additional cost. So right now the plan is to buy the space black series 4 come September. Of course, we'll see how what actually is released may change that.