I don't think Apple sees itself competing with Samsung period. it's a totally different ecosystem, totally different OS. the folks that run off and buy a Samsung over the screen size when they would actually rather have an iPhone is probably very very small. Apple did a larger phone because it was a logical decision. And yet made two sizes because they knew not everyone is so hung up on size. Even if they hadn't made the Plus, they would likely still have gotten a lot of sales because the size isn't that huge of an issue for most folks.
Apple doesn't compete feature for feature, but they definitely see Samsung as a competitor in terms of the phone. They sued them for copying them, though some of that may have been Steve Jobs' personal vendetta.
I agree that with the Watch, Apple seems to be viewing this as a brand new and wide open market there for the taking, rather than a market where they are competing with Samsung, Lenovo/Moto, or LG.
In any case, Samsung's back in "photocopier" mode with the Galaxy S6, and it will probably be another hit from them. While unrepentant fanboy sights like Sammobile will hem and haw about how Samsung had watch phones back in 2009 and "smartwatches" as early as 1999 (and of course the Galaxy Gear), my guess is that the folks in Suwon are closely monitoring the Apple Watch, strident comments from their execs about how Apple is "following" them into the market notwithstanding. The rumor is that Samsung has been working on a round watch with a rotating bezel, and may be partnering with a watch maker or luxury brand. That may be true, or it may be vaporware of the likes of every Apple product is worse than some forthcoming Microsoft product (as Macalope likes to point out).
It's likely that Apple accepted more pre-orders in one weekend than Samsung has sold watches to date. Actual "sales" will be tricky since they'll only be counted when they are actually received by customers over the course of the next 2-3 months. I expect we'll see every "smart watch" manufacturer, whether a tech company, or a traditional watch maker, release a smaller watch geared toward women, while before Apple the trend was toward bigger devices.
Interestingly, there still is a fascination in the Android Wear world with round watches, with Huawei entering the scene, but if Apple shows it's possible to pay homage to the mechanical watch without aping its design, then we may see more rectangular designs come back, as well.
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It remains to be seen but we're in the very beginning of the product category, kind of like iPod was.
So I suspect that we'll see huge changes in form factors and sensor technologies being incorporated in a few years.
In addition, since this device is meant to be seen (it's on your wrist), I think that it might go out of style more quickly than a phone might.
Bands will change, and styles will come and go. I don't think the basic shape of the watch body will change much, though we'll see tweaks just as we do with phones.
I don't see Apple releasing an "Apple Watch 2" and dropping the price of the "Apple Watch 1" like they do with the iPhone. I think they'll be more likely to make on-the-fly enhancements like they do with the Mac line. They don't sell the 2013 Retina MacBook Pro for $200 less than the new one. I think they realize people will want to keep watches for a few years before replacing them.