Ok my two cents, worth probably less than that.
ME3 - great game that we all have come to know and love. But now two years out is a bit dated and dull. Its game engine is showing its age as are the graphics. But playability is still pretty amazing. But remember, when it first came out, people complained about the MP, about the story game, etc., etc.. They honed it over time and it became better. Most people playing the ME3MP from the Beta would say the game is pretty different from when it was first out.
Destiny - Impressive graphics and decent gameplay. I think it will get better since it is only the first release of the game. They have made a 500 million dollar, ten year commitment to that game. So I think (I would hope) it will get better over time. It has some features that are better than ME3MP and some that are worse. But it is new and will need some time to modify things. Etc. I especially hope they work on the weapon mods and class features. Maybe add some more classes and enemies. Remember we originally only had a few classes and three enemy types for ME3MP.
So while the games are both "Space Operas" and MP/Story based, they really are different games. One doesn't have to be better than the other. You can like both. Or even neither (but I would wonder why you would be posting here then).