Under a helmet, the X5 would be fine. It's not good under a hat that covers your ears to keep them warm. With the stock tips the microphonics of the X5 are very similar to the CX300. Those are the only 2 in ears that I've used that had microphonics with a slight breeze blowing over the cable. The microphonics on both are lessened by putting the cable over your ears, but neither are designed to do that so it's a little awkward, more so with the X5.
I listen to all types of music with both. The X5 sounds better with everything because it just sounds better.
One thing to consider, if you get wacked on the side of your head, it's going to hurt a lot more with the X5 because it will get jammed down your ear canal. The body of UM1 won't allow that to happen with it.
Do you really want isolation when running or mountain biking? If you run in the street, I wouldn't want isolation. Likewise if you're riding with other people. Also, at least for me, fidelity isn't as important when I'm doing something active or in a noisy environment. I use my IEMs for the gym, commuting on the subway, and plane travel, all noisy environments where fidelity doesn't matter.
Unless you need isolation, I'd suggest something like the KSC75.
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