When comparing BrowserStack vs Endtest, 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 Endtest is ranked 3rd. 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 Codeless automated testing
Endtest allows you to create automated tests for web applications, directly on their cloud infrastructure. No coding skills are required.
You can also choose the action "Execute Javascript" for a more flexible test run.
Pro Running your automated tests on their cloud infrastructure
You can easily run your automated tests on their platform, just by clicking the run button.
Pro Designed for Continuous Integration
It has an API which allows testers to run it on Jenkins or Team City. Other than that, it has a built-in test scheduler to allow testers to preset test executions.
Pro Now supports mobile testing
Endtest has added a new feature which now supports mobile testing for both Android and iOS (.apk and .ipa).
Pro Option to export your automated tests in different formats, including .EXE
You can export your automated tests in a variety of different formats, and run them on your own infrastructure. The formats are either in Python file format or executable file format (.EXE).
Pro Now supports a wider variety of browsers, operating system and devices
As of March 2018, Endtest can now support multiple platforms (Windows, MAC, Android and iOS), Operating Systems, and more than a handful of devices for both Android and iOS.
Pro Super fast and affordable.
Pro Premium version is relatively cheaper than most in the market
Their pricing plan ($79/month or $650/year) includes team plan upgrade, which means the whole team, regardless of the number of users, will only need to subscribe to the $79/month or $650/year plan. This includes unlimited test runs, test exports, recordings, etc.
Pro It's mostly free to use
The free plan covers a lot, you can have up to 3 test suites, and you can run your automated tests as many times as you want. The pro plan includes some very advanced options that you might need only if you're working with a team (for example, the collaboration feature, schedule test run, API for Jenkins, etc.).
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 Doesn't support a wide array of browsers and/or operating systems
Because it's still in its infancy, it doesn't yet have a huge variety of browsers and operating systems.
Con Free plan has limited time storage for test suites
If you are using the free plan, keep in mind that the test suites created there are scheduled to be deleted after around 2 weeks. Endtest will send you a notification email the day before mentioning "Test suites from users who are not on the PRO Plan get automatically deleted after some time," and offering for you to upgrade to PRO.
Con Codeless and locks you in rendering your efforts useless if you want to switch
Con Bad UI
