Hi Andy,
I own a an assortment of excellent headphones (Hifiman HE500, AKG K550, ATH M50, UE6000, Q700) and have experience with the M50 (not the X version) and K550. They are both good headphones but I think the K550 is better. Firstly, the soundstage is probably the largest I've heard on a closed headphone, and when used with a decent DAC it is very detailed. The mid-range is also a delight! Compared to the M50, (I know the M50X may be improved) it is much more balanced and musical. The K550 bass is present and is nicely proportioned and I don't personally find it 'lean'. It actually does sub-bass too due to it's large drivers (which the M50 can't). However, If you're a bass head and want pure bass impact then you may want to go down the M50 line as it hits much harder. Personally I think the AKG is musically on another level. I must add, while I haven't listened to too many Shure phones I know they also have a reputation for great detail so that would be worth a look to (just make sure the sound isn't too bright as I've heard people say they can get fatiguing).
Below (in the links) are the frequency plots for the K550 and M50X. You can see the K550 goes deeper (top left graph) and has much tighter bass (see square wave). If it were just between the two I'd get the K550. If you're unhappy with the bass you could always add a touch of EQ to the bass in your music player (I don't think lack of bass should be a problem anyway). Other things about the K550: It is the most comfortable headphone I own (needs a big head though to get a seal though, this is actually crucial for bass), it's very light in weight, and it's very easy to drive (good sensitivity).
http://www.innerfidelity.com/images/AKGK550.pdf
http://www.innerfidelity.com/images/AudioTechnicaATHM50.pdf
In summary, they are both good phones but I'd choose the K550 any day of the week (vs original M50). Good luck and try and demo them all yourself!