If you can produce a playable demo, Steam Early Access seems to be the safest way to go for a developer. Once their build for SEA is approved and published, the developer is under zero obligation to deliver anything beyond that. Steam Early Access backers are expected to be satisfied with the game as it exists at the moment they purchase it; anything beyond that should be considered a bonus.
Now, obviously having the developer abandon their SEA product is not the result backers hope for (nor would it be a good move for the developer's future projects), but the point is the developers don't have to worry about explaining why they didn't get stretch-goal features in, or deal with complaints about someone's missing Kickstarter pony reward, or what text is on what box, etc. It's just a lot.. simpler.
Of course, losing 30% revenue off the top to Valve sucks. But I suppose it depends on how much money is being brought in by SEA versus Kickstarter.
There's also an argument that Kickstarter generates publicity which makes a game successful on Steam Early Access, but I suspect promising projects on Steam can generate their own momentum on Steam just fine without Kickstarter's help. In fact, too often lately projects are showing up on Steam Early Access with pricing that is shocking and angering would-be SEA backers, all in the name of keeping things "fair" for the Kickstarter backers and their pledge tiers. The result is a forum filled with bitter postings over prices, which is the first thing a potential buyer sees when looking for reviews and opinions. It turns the whole "word of mouth" benefit into a nightmare, and I'm really starting to think Kickstarter does more harm to titles on Steam Early Access than it helps.