MG7s lack the deep layered lush highs and mids that the CKM500 have. If all you want is sub bass than the MG7 is what that is about. Otherwise they are just not that great even with a %50 discount.
WS55. Similar sound tuning. But for the cash I would actually get a JVC HA-S500. Look up that thread. The HA-S500 is an upgrade to even the WS55 for less.
Look forward to your take on them. The CKM500 is an excellent all round earphone. I am sure you will like.