1. IEMs or canalphones or in ear monitors rely on a tight fit to reduce sound by about 28db. Without a tight fit the phones sound thin and lacking bass
2. Headphones such as the PCX250 which are small supraaural closed back cans with a noise phase inversion circuit can reduce certain sound frequencies by about 25db, but are not as good at blocking all sounds as a good fitting IEM
3.I have both these phones listed and many many others to listen to.
4. Be carefull where you buy the Senn CX300, they are the most faked and counterfeit product in the known universe of sound reproduction.
5. The CX300 come with 3 different sizes of silicone tip some you can work on the fit. If the fall out, route the leads over the top of your ears.
6. If you can spring for the CX500 or CX400, you are less likley to find them being faked, they have a better sound in that the bass is less wooly and the teble is less splashy and grainy and the come with an additional set of bi-flange tips which can be better for some people. (Yes we have the CX500 as well)
In summary. Good fiting IEMs ( or buds as you called them) are much better at blocking external noise across all spectrums than any supraaural noise reduction can.
Full sized circumaural cans with noise reduction such as PCX450 are better at bloacking sound than smaller PXC250. Heavy and make your ears sweat, look bad worn in the street
Good fitting IEMs are better than both but take some pratice to get a good fit.
Consider Phonak Audeo PFE IEMS which cost more but get great reviews.
Consider CX500 above the CX300