While the S10+ competes with all of the big-screened phones on the market, the S10 with its emphasis on compactness, has a bit of a narrower set of natural competitors.
The iPhone XS is the original archrival and while this dilemma is often decided on grounds of ecosystem preferences, some other considerations could come into play. The S10's ultra-wide camera has obviously no answer from the iPhone, but when it comes to selfies the S10 is slightly better too, even for the same number of modules. The Samsung phone lasts longer in our battery tests, charges faster, and has a brighter display out in the sun. It's also substantially lighter, which is likely important if you're eyeing the non-Plus/Max versions.
If pocketability is the top priority, the Pixel 3 is still that one bit ahead of the S10, though at the expense of screen estate. The Google phone is a couple of cameras short on the back, and one has to wonder if the versatility of three modules doesn't beat having a single awesome one (it probably does). The Pixel does have two selfie cams and one is a true wide-angle unlike the Galaxy and its sort-of-wide/cropped-in trickery. Then there's the vanilla Android vs. One UI conundrum.
LG's G8 ThinQ will come with a heavily-modded UI like the Galaxy, and we know how both skins have passionate haters. That aside, the G8 should pack a triple camera setup like the S10's (unless you're in a market where the G8 will have two cameras, because LG) and a nifty ToF camera to complement the 8MP selfie shooter, though we're still leaning towards the S10's dual pixel 10MP cam. The S10 outclasses the G8 in terms of design, but we can't yet comment on the objective stuff like battery life and display quality.
The Xiaomi Mi 9 isn't quite as compact as the S10, but if you can live with the extra bulk, you'll be rewarded with better battery life, an autofocusing ultra-wide camera as opposed to Samsung's fixed focus one, and substantial savings - you can have two Mi 9s for the price of a Galaxy S10.
The regular-sized Galaxy S10 is the one to have this year if you're not entirely convinced you want the extra screen the Plus delivers. Sure, the bigger phone will come with a battery life advantage and the added depth sensor on the front will make for better selfie portraits. But we feel it's not quite worth it - unless, again, screen size is a driving factor.
The S10 makes a lot of sense as a replacement for the S9 in a way that a yearly upgrade rarely does - two more cameras on the back and a new one on the front sounds like reason enough. If you're coming from the S9+, it's less definitive, because last year's Plus still has better battery life and is only missing an ultra wide cam.
In the end, we feel like the Galaxy S10 is the superb phone we could have predicted it was going to be. Years of refinement have brought us to a place where groundbreaking changes are unlikely, but as incremental upgrades go, this is one of the more meaningful ones and it's easy to recommend.