Well for one the WS99 is more versatile in that they can be worn out doors. the A900X is the exact same size as the AD900X..A900X the pads are stiffer pleathers which some of the guys on the A900X had an issue with. They get more comfier over time but comfort wise I would say the WS99 beats them as well.
Sound wise there are some similarities in the sound of the A900X and your AD900X but the AD900X has a newer revised driver that has the better balanced sound with better resolution in details and that awesome stage.. The WS99 isolates better and has the more fuller sound over the A900X. Smoother more forward mid range on the WS99. The bass end both are excellent but the WS99 has the edge here.
I would go for the WS99 it isolates better, weighs less, more comfier, has a very nice full sound and can be used as a portable headphone. Not so much the A900X. The highs are about a wash on both cans though I think the overall sound because of the dual air chambers on the WS99 are deeper be it the highs mids and bass... The A900X are wider in stage but not by a significant amount. You want stage you already got the one for that. Depth in sound goes to the WS99.
The A900X is a good deal for the price you can get those for but for roughly the same price I would go for the WS99.. Both have a great balance but the little things on the WS99 makes them a real nice compliment to AD900X.