Apples to oranges. They are a bit different then each other in sound signature. I would call both quite capable products. Neither really fault in any major way. Both have excellent frequency response range, smoothness of response, presentation, accuracy, etc... They each present the sound in their own flavor. If you're a bass fan, you lean a bit towards the IE8. If your a mid and treble fan, you lean more towards the TF10.
The IE8 has a bigger, more spacious sound stage, but the TF10 is well sized too and offers a greater sense of layering and depth. The IE8 is a little more laid back in presentation, a little softer on note and energy. The TF10 has sparkly treble that comes across limitless in extension. The IE8 does this same effect more so with bass and gains a good visceral sense because it does retain response sensitivity down to the bottom. The TF10 is well extended but drops sharply just before it would gain the visceral sense. It doesn't come across lacking, but the roll off is just early enough to not present the 20Hz to 40Hz range with equal SPL of the higher frequencies. I consider the TF10 to be more refined, more balanced, more even then the IE8. Both suffer from their physical design when wearing them. The TF10 is wide near the tip and can limit insertion and seal. You need a tip that fits you well to make the TF10 work well. The IE8 is somewhat wobbly in the ear. The over-ear guides help some, but I'm not a fan of the ones that come with the IE8. I think Phonak Audio has a more ideal design for over the ear wire guides. The bass knob on the IE8 is somewhat pointless. It simply does the wrong thing. Well it does the right thing but just in the wrong place. It's just that the bass bump starts up in the lower midrange. The bass knob starts affecting output down at 100Hz. It can't help the lower midrange and midbass bump. It pretty much makes the bass even (turned up) or anemic and lacking in extension (turned down). Either way, vocals are bass heavy and warm, the bump stays there. You pretty much just turn it up and leave it.