Which Motherboard should I buy if I want to use a PCI-E M2 Card for booting

[kevin.karthaus97]

Hey guys,
I'm currently building a new pc from scratch and since my old one only had hdd's in it, I'm looking for a huge upgrade.

I'm wondering if I may use the build in M2 slot on the Mainboard or rather getting an m2 to PCI-E adapter.
Which option will give me the higher performance and will it be working for booting?

Here are the specs:
-i5 9600K
-16 GB DDR4 3000 RAM
-Samsung 970 EVO 500 GB, Solid State Drive
PCIe Gen 3.0 x4, NVMe 1.3

Here are the mainbords to concider:
-MSI Z370-A PRO
-MSI Z370 TOMAHAWK
-GIGABYTE Z370P D3

The last one actually got an PCI-E x4 slot, that's why it was my favorite.
However it looks like there are only x16 slots on the board but they are shown as x4.

I want to use this connector for the M2
-Aquacomputer kryoM.2 PCIe 3.0 Adapter

 

[Tanyac]

Use the M.2 adapter on the motherboard. Generally at least the first M.2 slot will take its PCIe lanes from the chipset, which means your graphics card will have the full 16 lanes for it's use (not that x16 vs x8 would yield any significant improvement anyway).

Don't get an adapter; you don't need one for these boards.

Personally, I'd go for the MSI. Better features, such as 2 x M.2 vs the Gigabyte's 1 x M.2