Hi there,
Both of those slots accept drives that are the M.2 form factor. There are SATA drives and PCIe drives that share that form factor. SATA drives will be limited up to about 570MB/s sequential read and write speeds, whereas their PCIe counterparts can do about 3,500MB/s sequential. The other thing you'd have to worry about is IOPS (inputs/outputs per second). IOPS determine the amount of data the drive can manage. Higher is better.
The numbers you see (2242 / 2260...... etc.) are just the dimensions of the drive. M.2 drives come in different lengths. 2280 is the most common, and is 22mm x 80mm (looks like a stick of gum)
So... The short version is this:
- M.2 is a form factor for SSDs., and they come in different lengths
- If you have plenty of cash to burn, get a PCIe drive, as they are generally quicker.
To specifically answer your question about boot drives,
any SSD is great as a boot drive, but each slot should perform the same.
If you have any more questions about this, let me know. Hope this helps!