Purchased the Meshify C Tempered Glass recently, fell in love with it instantly even though I'm not getting the rest of the components for another month (it was on sale). In terms of airflow, it looks like they'd be about the same. Both cases also have 3-pin fans included instead of 4-pin, meaning they'll be running at 100% constantly. That might contribute to a bit of noise.
What I'm noticing is that the Meshify C is much friendlier towards having multiple SSD's (Up to 5 SSD when not using any HDD, highly recommended. 2 SSD would take up the HDD brackets), while the P400A is geared towards having more HDD's (Up to 6 HDD with space for 2 SSD, but the additional brackets are sold separately, and would mount in the front side of the case). If you don't intend to use any HDDs, the Meshify C would be your best option here.
The Meshify C also has a bit more room for cable management in the back. There's a bit of an indent behind the front panel that gives optimal space to run the case cables down the side, as well as any other cables running through that area. The back of the Meshify C also has many more zip tie locations than the P400A, giving you more options for routing.
The Meshify C's customization options are also very very nice. There's a detachable panel on the bottom shroud, which allows up to 2 additional fans to be installed, one on the front, and one on the bottom (this would replace your HDD cage if implemented). The panel that the SSDs attach to is also completely removable, if you don't intend to use them, or if you want the SSDs in the drive cages. This frees up space behind the motherboard if you need that for any reason (realistically, the only benefit would be access to the CPU cooler mount, if one is used, which would let you swap coolers without detaching the mobo from the case). I've also seen that the front panel mesh can be removed entirely, and swapped out for one in a different color (sold separately).
Ideally, I'd recommend the Meshify C, however the P400A would absolutely get the job done too. If you're on a strict budget, going for the P400A would still be a great choice!