Go for the Msr7b if you want a better treble extension, clarity, and detail retrieval.
Go for the 99s if you want better bass extension, with a more meaty, weighted sound.
Both are fairly forward sounding with excellent mids. Again, the 99s being meatier, but I wouldn't call the mids on the Msr7b thin either.
I slightly prefer the 99s, as sub bass and drum work is more present, and electric guitars have more crunch, but less sizzle than with the Msr7b.
The Msr7b has a wider, more spacious sound. Instrument separation is really good on both, but I'd give a slight edge to the Msr7b. The wood cups on the 99s provide a more organic timbre. I think the Msr7b benefits more from a balanced connection, as a balanced connection provides better clarity, separation, detail retrieval, and treble extension. All the things that the Msr7b already does bett than the 99s, without a balanced connection.
You really can't go wrong with either imo. They contrast yet compliment each other so well. I don't see me parting ways with either pair anytime soon.