Depends how badly you want the stainless steel - and how much disposable income you have combined with how willing you are to part with that disposable income for the stainless.
Currently, I consider the watch a disposable/short lived device (unlike a traditional watch). I've gone through two watches with the same iphone so far. This may change once the feature-set and performance settles down, but right now i do not see the point in spending a lot more money for stainless steel when i'm going to replace it inside 2-3 years anyway.
With that viewpoint in mind, from my particular perspective the stainless steel is a complete waste of money at the moment. I have a black milanese loop that looks fine with my space grey s2 or s4.
Again though, like i said - if the feature set and performance settles down and apple provide a service/repair program for watches older than a couple of years in the future, that may change.
As far as durability goes - my S4 sport is un-marked. My S2 sport has a cracked screen (as a result i use it for exercise or if i am doing something where there's more of a risk of damage, e.g., working on the car, etc.), but i am pretty sure that is apple-defect related rather than from impact as the watch has never sustained an impact in that area, and there are no other marks on it.
Apart from that damage, both the S2 and S4 took a few knocks from time to time and are both otherwise un-marked. As i said, i fully believe the S2 damage is a manufacturing defect as there is no chips or scratches, etc. apart from an edge crack. The sport glass is a lot more durable than i expected. I haven't abused it, and i do try to look after my stuff... but i honestly expected at least some scratches in the time i've owned them. I do not plan to buy Stainless Steel any time soon as i think the Sport is definitely "durable enough". And even if i do break it, i can buy another one, likely the next year's model and not be too far out of pocket vs. the cost of the original stainless watch.
It’s entirely up to the individual and what they want. You can’t really answer this factually.
I think the OP is more concerned with opinions, as yes... "worth it" is entirely subjective.
🙂
Having two Apple watches, one constantly on charge and ready to go would be sweet!
This is another reason to go Sport
😀 Upgrade more often (for same outlay over time), keep the old one as a secondary. I'm doing this with my S2/S4 combo at the moment.