I've got an ATK 300 FL and a StingRay 4FL. Should make a dead-on comparo, but my Ray is modded, BassLines PU and innards. However, I have compared my Ray FL to a stock Ray FL. Stock has more sizzle, which I am glad to do without.
My ATK is the best MM-inspired ax I've every played. Not talking about appearance-rip-off 'clones' here, just things like the Ray, the ATK, the GnL Climax,, etc. It sounds terrific, hotter than a stock Ray but smoother [meaning less sizzle, less 'brittle']. So did Ibanez beat MM at their own game? Not really. As others have mentioned, ATKs are great boat anchors and it's not too much of a challenge to get humongous tone from humongous 12 or 13 lb slabs of wood and wire with thru-body stringing.
If you really dig your EB MM [I assume you'll get into them], the ATK is a very compatible back-up if you don't have to wear it for hours on end. Maybe you'll want to defret it. Most players wear their fretless less than 25% of the gig.
I'm gonna get flak for this, but as a player of both, I can't tell you that you'll get a several hundred dollar improvement in sound and playability. They did a really great job with the ATK so the difference won't be huge. But you can find StingRays under 9.5 lb. The tuners on my ATK are not so great [not worn, just mediocre], and the wood you see thru the burst finish is truly ugly. The MMs are plainly a better bass, but knowing the ATK as well as I do, there's no urgency to upgrade. It's not some entry level turd. You'll part with it or retire it when your sick of the weight.
But if you want a Bongo...you need a new bass yestrday already.