I'm fairly certain I want the SS, because the Sports doesn't look like a serious person's watch. If I'm spending hundreds of pounds, I want something I'm going to enjoy owning and keeping for at least two or three years. And I'm fairly sure I want the 42mm because the 38mm just seems a bit small and the 42mm is no bigger than my current watch. (Some photos make it look really big but it's impossible to judge how big people's wrists are. The various cut-outs I've tried seem to make it look OK.) Plus the bigger screen is probably better for my eyesight and there might be some battery advantages.
I also want the 42mm because I like the leather loop. And here's where it gets difficult, as the OP says.
None of the straps are ideal. The Milanese loop has a lot going for it, but there are also some question marks. The leather loop seems to look great but my wrists are on the cusp of the two strap sizes and it's not obvious which one will look best. The basic plastic strap looks great in Apple's renders, but looks like a dust magnet in pretty much every photo I've ever seen. Besides, it's plastic. On a £519 watch.
The obvious solution is to take the OP's advice and wait until I can see the thing in real life. But that's not particularly convenient and, besides, I'm impatient!
I'm currently favouring the following strategy. Buy the SS with plastic strap. Separately buy either the Milanese loop or one of the sizes of leather loop. If it looks and feels good, keep it. If not, send it back and get one of the others.
This way I get to keep the watch, rather than having to send the whole thing back. And the plastic strap might have its uses sometimes. I might even like it. Or I could almost certainly sell it. (Actually, because you get two plastic straps when you buy a complete watch, I'll definitely sell one of them at least, and mitigate some of the loss I've made in buying the second strap rather than just buying, say, the leather loop as a complete watch from the start and risking not liking it when it arrives.)
From what I've seen on these forums, I think Apple is going to have vast numbers of cancelled orders and returns. I hope they've prepared for that logistically.