I'd venture to say that my Andromeda is nearly as engaging as my HD 800S, if not more.
There is an immediacy to an IEM sound that I don't think a full-sized headphone can capture.
In terms of technicalities however, I'd say that full-sized headphones will almost always win out.
I recently read somewhere where head-fi member compared the Legend X to mid-fi level full-sized headphones.. the price difference is something to take into consideration though. For instance, an HD 650 could be bought for under $250 .. while you'd be lucky to find a used Legend X for under $1,900.
How often will you be using your full-sized headphones? An IEM? Most people tend to use their IEM much more and find their expensive full-sized headphone collecting dust. Are you the type of person that likes to escape from the world with music? Or does music just add to the experience of what you are actively doing, walking, etc? All of these things come into consideration in my opinion. I didn't think much of these things when I first came into this hobby.