When comparing BrowserStack vs Sauce Labs, the Slant community recommends BrowserStack for most people. In the question“What are the best cloud-based cross-browser testing services?” BrowserStack is ranked 1st while Sauce Labs is ranked 6th. The most important reason people chose BrowserStack is:
IE, FF, Safari, Chrome and Opera come with additional web development browser extensions installed, such as Firebug or YSlow.
Ranked in these QuestionsQuestion Ranking
Pros

Pro Good selection of developer tools
IE, FF, Safari, Chrome and Opera come with additional web development browser extensions installed, such as Firebug or YSlow.
Pro Easy to use UI
Pro Huge variety of OSs, browsers and devices
There are over 200 desktop/mobile/tablet browser-platform combinations possible.
Pro Integration with CI services
Pro Get screenshots across many devices
Can request screenshots from many devices for comparison.
Pro I can easily test websites on my local development

Pro Free JS unit tests for open-source projects
Developers are advised to use the contact form. The JavaScript API is free for open source projects. Unclear whether automated testing with Selenium is also free - apparently not?

Pro Selenium automation integration

Pro Run concurrent tests in parallel
Open source project accounts are limited to at most two concurrent VMs.

Pro Might be able to test on real iOS devices
It's not very clear if real iOS devices are available for automated testing.
On one hand, https://www.browserstack.com/mobile-browser-emulator is all about emulators for both Android and iOS.
On the other, https://www.browserstack.com/automate says
"Test across real iOS devices and Android mobile emulators"
Then again, https://www.browserstack.com/list-of-browsers-and-platforms?product=automate lists "real iOS devices".
Pro Huge variety of OSs, browsers and devices
There are over 300 desktop/mobile/tablet browser-platform combination possible.
Pro Test on Real Devices
Sauce bought Test Object and now offers automated and manual testing on real devices in public and private clouds.
Pro Selenium automation integration
Pro Integration with CI services

Pro VMs are destroyed after each session
No danger of data leaking to other users who might be using the VM, unlike with BrowserStack
Cons

Con Sessions are rejected instead of queued up when all VMs are in use

Con Security incident in 2014

Con Real Apple devices are unreliable as of August 2015
Real Apple devices are a new feature that BrowserStack has introduced. At the moment (August 2015) there are unreliable. Excerpt from an email reply from BrowserStack support:
we provide real iOS devices (Beta) and iOS simulators for Selenium browser testing on Automate. Since these devices are in beta, you might encounter a few problems while running tests on them.
To avoid this issue, we would suggest you try running your tests on iOS simulators.

Con No Linux support

Con Uses Android emulators for mobile testing

Con May be unreliable
The same test may succeed for some users and fails for others. Apparently there are problems with the test client running on Windows.

Con No longer free for CodePen projects
Con Re-use of VMs across different customers
Con Very slow
Can take several minutes to start a VM, which is much worse than competitors
Con Graphical artifacts
Frequent screen tearing and other graphical artifacts make it harder to use.
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