Both?

Lavender event continues until November 10th. Not knowing what the the next event entails is the problem. If it's a medal or raid boss farm then you're golden, but if it's a collection...

It depends on if you want a Trick or a Treat.

Rush the Lavender one first then try to get as many pumpkins as you can from the Halloween one afterwards.

got all ampys & manyus using pumpkin i got during 1st phase.
now i fully aim for Lavender (got her & all blooms already)

with all i got now. im back to farm 2-1 again =w=

I just did enough pumpkin runs to get to 500 life crystals for a rainbow skill bloom. The pumpkins I spent on 11 DEF ampys, since I already had the ATK ampys and all the manyus.
So, now I'm going to focus on Lavender and getting her some blooms (which are gladly much cheaper).
Tomorrow is red/blue manyu day though, and I could still use a LOT of red manyus.

Lavender, no question. I'm even working toward a second one to compliment the one I got from her original event, because she's good even without full skill/equipment. Though I must say after finally seeing her story scenes, she's a bit of a bitch.

The Halloween event would end as soon as tomorrow (November 1 iirc) and the Bremen Reissue Shop at November 10.

The plan in my case is simple. Finish the Halloween first and buy as many ampys as I can. Each one costs 550 pumpkins and there's a dozen for each type so it's quite difficult to buy all of them.

As for Lavender's Reissue Event, 3-4 days are enough to buy everything. Even if the new event comes along, it wouldn't be difficult to dual farm them. The only one with hefty price is Lavender herself with 2000 sealstones. The rest are jiffy.