Dat staff.

If I remember correctly, you have the following speed boosts:
Booster (-2)
AYA (-1)
Body Boost (-2) (Is this independent of AYA's speed boost? I'm assuming so)

So that's -5, assuming Body Boost is always on (Is that a thing? Can people do that? You'd need a lot of buff duration), which brings you to Faster(3) with a Slow(8) staff. Finishing Blow (990ms) + Sweeping Staff I (240ms) + Sweeping Staff II (300ms) is 1530ms at Normal(6), 1440ms at Fast(5), so 1260ms at Faster(3) and 1170ms at Faster(2).

So, an Elemental Staff would be 7.6% faster than a Slow(8) staff, so, for a slow staff to be better than an Elemental Staff, you'd need to the difference between the magic attack on the staves to be greater than 7.6% of your total magic attack, and 23% if you ignore Body Boost (Slow(8) would become only Fast(5) in this case).

If we look at the duration of Body Boost, it's 90 seconds with Union Aura, with a 120 second cooldown, so 75% of the time, you'd need an increase of 7.6% magic attack and 25% of the time, you'd need an increase of 23% magic attack, which averages out to 11.45% more magic attack.

I would say the elemental staff is a good investment, unless you're planning on getting a Fafnir staff or something. I'm not sure what your stats are, but I'm guessing it'd be difficult for a slow staff to match the elemental staff, but again, it depends on what your total magic attack is.

Body Boost is 90 second duration and 90 second cooldown actually, since hypers you can have Body Boost on permanently, providing you are very attentive to when the buff ends.

The elemental staff has more magic attack than the Fafnir staff, I think, plus it's fully tradeable. So once Blaze Wizards get revamped it will be able to use it too. (and likely gain 5% extra damage, since this is a Poison staff)