You're gonna have to lay out what kind of music/sound you like.
CX300-ii has more bass than E10. E10 has much lesser bass (though generally, I'd say both are on the bassier side). The E10's bass is much less bloated. It's much tighter.
Then there's the mids. E10's mids are a lot smoother. However, some genres benefit from that, some don't.
E10's highs are nice, CX300-ii's aren't bad either. Slight edge to E10.
For me the CX300-ii's large bass kinda spoil the rest of the frequency range.
Then you gotta figure out what kind of sound you like in terms of spaciousness (I many not be using the most accurate/appropriate vocabulary, if someone more experienced is on this thread, correct me).
The CX300-ii's sound is very intimate (due to the slightly bloated 'overbearing' bass). These make the CX300-ii's very immersive. Some genres benefit from it, some people like the whole 'immersive' sound. However, this intimate, immersive sound comes at the cost of layering. This is where the E10 excels. The sound is wide and airy and roomy. The silence between sounds is nice and black. There is a much better sense of spaciousness. Different instruments, different 'layers' of sounds are well presented. Personally, this spaciousness is what sold me on the Soundmagic products. While it will lack the intimacy of the CX300-ii, it'll gain the spaciousness. Also, if you're using these for gaming, the E10's are significantly better.
From a sound quality perspective, amongst the two, I'd go for the E10.
There's other things to think of too, beyond just the sound quality:
(1) The market is littered with CX300 fakes. Be careful. As long as you've got a certified seller, you're safe.
(2) The Fit/Comfort. The CX300-ii is a dream. It fits amazingly well. You could fall asleep in it. The CX300 (and the similarly shaped/designed EP630) are amazingly comfortable. Period. The E10's are a whole other story. The housings are tubby, kinda uncomfortable on some ears. Could work fine with the right ears, but there's absolutely no denying that the CX300-ii are the epitome of comfort.
(3) Build quality. I'm not sure you'd care much for this, they're budget iems, you could use it for a few months and throw them later, no harm done. But if you care, the E10's come with aluminum housings and a twisted 'weaved' cable. Much better than the plastic and rubber on the CX300.
Now. Other options.
Firstly, there's the E30. Basically what the E10 does to the CX300-ii (reduce bass, tighter bass, better sq, spaciousness), the E30 does further. Even lesser bass. Where the E10 are bassy, the E30 are more balanced (some people still call it a hair bassier than neutral). These are not as bassy as the E10. That makes them less suitable for a few genres. The mids on the E30 are marvelous. Extremely smooth. The treble is great. Sparkly with no spikes or sibilance. More spacious and significantly better layering when compared to the E10. You might find its shape/shallow fit bothersome, though.
Other option - Signature Acoustice C12. Made in India. Its worth being proud of. Now how I described the E30 being a better E10 of sorts, think of the C12 as a significantly, significantly better CX300-ii. The same bassy and intimate sound, but quality bass. Deep bass. And it has a lot more 'dimensionality' than the CX300-ii. Also, great build quality and accessory set. With this in the market, personally, I think there's no reason to buy the CX300-ii. Well, except for the CX300-ii's comfort.
Others to consider. (disclaimer: no personal experience on the following)
Kilpsch S4. Had an ears-on on a friend's pair. Not worth the price. However. These are often found on discount. That's when you hit the button. They're worth it if you find them at around $50.
Audio Technica CKN50. Never heard them myself. I can't imagine myself liking the fit. But there's a whole thread on head-fi thats a fan of these and the higher ckn70. Generally, you can't go wrong with audio technica. But good luck finding the reviews, they've got a billion different iems for different sounds, and a billion different versions for each region.
Brainwavz M1/M2/ProAlpha - They're good. Worth the money. Not exceptional for the price, but generally likable sound. Good build quality, good comfort. Nothing wrong with these.