[JakiChan]

I've got a pair of ER6is that are my gym phones.  Driven straight off an iPod nano.  I've loved their sound for years.  I've had a few pairs.  You can still get them off of Amazon, but clearly they're on their way out.  
 
Now there is the MC5.  I know, BA vs DD and all that, but I'm not sure how to quantify the sound.  My question:
 
If I've been happy with the sound of the ER6is should I be happy with the MC5s?  Will they sound better?  I like plenty of isolation, decent (but accurate) bass, and a clean sound.  I could also spend the extra on the HF5s, but maybe they're overkill for the gym?

 

[supersleuth]

As a certified cheapskate, I say: if you're happy with what you get from the ER6i, why spend money making a lateral move as long as the ones you have are still alive? If you have the money to move up a class or two, that might make sense, but what you're considering is probably a waste.
 
I have only heard the MC5 (of which I am a happy owner). They're smooth and very neutral- to a fault, some (not me) would say. I gather from what I've read here that they may have less treble energy than the ER6i so you might well find it a downgrade, even as an eventual replacement when your ER6i dies.