WebdriverIO vs webdriver-sync

WebdriverIO lets you use your favorite testing framework (Jasmine, Mocha, Cucumber) and assertion library (Chai for Mocha). Other projects implement their own testing and assertion APIs, for example Nightwatch, Intern.

It should be mentioned though that v4.2.16 has an incompatibility with at least tap v7.1.2: stdout/stderr written during a run gets lost.

Chimp.js, is an emerging web application test framework that implements easy sync tests using WebdriverIO, CucumberJS and Chai. Features include:

• synchronous style
• built-in "widget framework" (an implementation of the PageObject pattern)
• automatically downloads dependencies (ChromeDriver, PhantomJS etc.)
• works with SauceLabs and BrowserStack (CrossBrowserTesting TBD)
• automatically takes screenshots on failures
• works on Windows in addition to Linux and OS X
• automatically produces boilerplate code for step definitions, which you can copy, paste and edit
• file watcher reuses the browser sessions and can run only the tests you tag, to maximize development speed

So you don't need to worry about promises

Run wdio config and WebdriverIO will generate a config file for testing locally vs. in the cloud, specifying the test framework (Jasmine, Cucumber, Mocha), where to find tests and store screenshots etc.

WebdriverIO has a plugin called WebdriverCSS that allows you to do cross visual platfrom/browser tests with an integration to Applitools.

WebdriverIO is accessible via gulp and grunt and even has a Sublime Text plugin for autocompletion.

selenium-webdriverjs:

driver.get('http://www.google.com');
driver.findElement(webdriver.By.id('q')).sendKeys('webdriver');
driver.findElement(webdriver.By.id('btnG')).click();

WD.js:

browser
  .get("http://www.google.com")
  .elementById('q')
  .sendKeys('webdriver')
  .elementById('btnG')
  .click()

WebdriverIO:

client
    .url('http://google.com')
    .setValue('#q','webdriver')
    .click('#btnG')

If you develop web applications with Meteor.js, you might want to use the xlovio:webdriver wrapper, because it's the Selenium binding behind the preferred testing framework (Chimp) promoted by the Velocity (Meteor's official testing framework) team for using BDD via Cucumber.

Service is provided by WebdriverIO which over comes the con of starting selenium server independently.
Reference: http://webdriver.io/guide/services/selenium-standalone.html

browser.get("http://foo.html", function() {
   browser.title(function(err, title) {
     assert.ok(~title.indexOf('foo title'), 'Wrong title!');
     browser.elementById('i am a link', function(err, el) {
        browser.clickElement(el, function() {
           browser.eval("window.location.href", function(err, href) {
           assert.ok(~href.indexOf('foo title 2'));
           browser.quit();
           });
        });
     });
   });
});

driver.get("http://foo.html");
title = driver.getTitle();
link  = driver.findElement(By.id('i am a link'));
link.click();
assert(driver.getCurrentUrl().indexOf('foo title 2') > -1);
title.should.equal('foo title');
console.log(title);
driver.quit();

If you've used the Selenium API before, you won't have to learn a new API.