you can also go for a pair of HD518s, they are cheaper than the HD558 and people over at head fi question if there really is any justification in the higher price.
This is irrelevant for the op: As for headphone sound quality it is worth noting when you move to high end headphones, as in really expensive, personal preference of sound becomes a factor, eg some may like perfect frequency response, some heaver bass some may prefer slight edge toward treble and so on, it is not just about tech specs and reviews.
I have used sennheiser for years and have had several of their headphones and the build quality is great. The HD518s I own have the same head band as the one in the previous pic and you would have to give it a serious beating to snap it like that, mine have been thrown/knocked around and they still work as new. My CX 980s (in ears) have also been used extensively and have been drooped, left outside in the wet (fell out my pocket), caught on door handles, been around the world with me several times etc and they also work as new, and have done for well over two years.
I agree with what PCunicorn said re on board sound and headphones, my headphones drastically improved the audio from both my onboard and soundcard (now I have an external DAC/AMP in preparation for the high end headphones I will buy soon, that also made them sound even better)
The momentums do seem to have good reviews but the hd5xx are more audiophile level and the HD6XX are def at that level and can be had for similar prices. Just checked the momentums and I couldn't justify spending $199 for an 8/10 product when the HD518s are 129 and 10/10 product, however they are two different products HD518s are aimed at home use whereas momentum's are portable. Similarly I found a pair of momentums at £200 yet HD650's can be had for £250, however you need to have the right gear to drive those cans
Basically OP are these headphones going to only be used for home audio/skype? If so, and you don't go with an integrated headset/mic, you will want to look for home audio headphones, and preferably go to a store where u can try them out to see if they will good for your needs (eg comfortable for long term wearing) also if you plan to also use them for portability be sure not to buy headphones with high impedance unless you have a seriously good music player to drive them, this will cause the sound produced to be quiet. The momentums will be fine @ 18ohms however the 38Ohms on the other set may be pushing it slightly (this is much less of a concern for pc use >50ohms will be should be just about ok, such as the HD518 or 558, HD6xx would be out of the question)
Overall the HD518 or 558, would be a good choice for home audio use, however the HD518s at least are a bit light on bass, but there are many options out there from several companies