So let me start with, neither is better than the other, they both have their flaws.
Warframe - It's a grinding simulator that takes space & ninjas, then throws they into an epic journey of constant battle till last person standing. As grindy it may be, it also has the *of course* "Pay to get stuff faster" side, where you can just pay to get whatever you want.
Destiny - Still a grinding simulator, but instead of space ninjas, it is "guardians" that protect the final safe haven on Earth from the "darkness". As it has nothing like Warframe that you "buy to get things faster", it does have constant DLC that come out, and lock you from certain things.
Conclusion - Both are wonderful games, but if you hate grinding for hours, play neither of them. If you're fine with grinding, give both of them a shot. You'll either play both of them, none of them, or one of them.
(Also, DE has always updated the game really fast, so it's gets new content a lot. But, at the same time, when Destiny gets it's new content, it is a lot.)
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Agree with all but the content bit, when ttk was downloaded onto our systems pre-release warframe also had an update, that update was just over half a gig larger than ttk, warframe has just released another free update since then, it is also larger than ttk was. Puts the content thing into perspective.
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I run a Warframe Group here on B.net. It's not really a clan, so you can still be a part of your war frame and/or destiny clan and you can join multiple groups at once. There are over 240 members in the group and we primarily use it for socializing about war frame, trading and recruiting. If you are reading this, I either already sent you an invite or you can message me for one. (Copy and paste is a life saver sorry haha)