Normally, yes, since characters innate AGL is from 6 (Ziggy) to 9 (MOMO), speed boosting accessories grants slightly better boost on speed. However you must remember that speed boosting accessories doesn't stacks with speed boosting ethers, If that is the case, AGL boosting accessories are better.
You can make a test, set the following skills on MOMO and make her your main battle character:
Now chose as team mates Shion, Chaos or Jr. which have innate AGL 8. You'll see that MOMO will act two times while these characters only act one time. That's because of her AGL is higher that double of their AGL.
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Fantastic. That's quite an in-depth explanation, really. But I doubt it would be applicable in my game style, though. I prefer a set team whom all the member more balance and powerhouse, with using speed effectively. So, I guess I would be using speed boost skill if I get the chance.