I own both. However I use the 990's for producing/mixdown and general listening to music and the 7506's for monitoring while recording SFX/foley (occasionally I do also listen music on them). With that in mind:
The 990's have a nice wide sound stage and come closer to the sound of monitors than the 7506. The 7506's sound stage is considerably more narrow as a result of the closed design as there is complete separation between both channels. This isn't necessarily a bad thing as the individual elements of a track don't feel cramped together. You can still easily distinguish indiviadual sounds and elements. The 990 have a more pronounced high end and the 7506 have more pronounced low end, but still nice and tight.
I prefer the 990's over the 7506's, due to their sound stage, but I have had the 990's for years now and know them well. Both are great headphones though. If your looking for headphones for just producing/mixdown then I'd get the 990's. Be sure your interface headphone amp can power them properly or get a dedicated amp. If you're also going to be tracking vocals/monitoring recordings then get the 7506's, the 990's will bleed through into your recordings. If you're not going to be recording with them and you don't have the gear to power the 990's properly or don't have the budget to do so then go for the 7506 too. Your mixes will most likely come out a bit too bassy with the 990's before you learn how they sound. Once you get to know them you're good though.
Another important reason I prefer the 990's is their comfort. The velour pads are very soft and the phones don't clamp too much (I don't have a huge head though

). The 7506's don't clamp either, still pretty comfortable but not as much as the 990's. The leather-like plastic (not sure on the exact material) pad cover is ok, but I've heard that after using them long enough it starts to flake and your ears and head will be covered with this shit. The pads of the 990 seem more durable to me. They can easily be replaced for both cans though. The leather headband on the 990's can be taken off and replaced too.
And the end of the day it's not going to matter much which pair you choose. Get to know them well and you'll be set.