system
Given the following post, respond as if you're commenting on Reddit:
human
I’m currently building my first computer. I have about $100-$120 should I go SATA 3 SSD or an M.2? If it helps I’m using a msi B450 Tomahawk Max motherboard. I’m new to this, sorry if this is a silly question.
gpt
M.2 is just a form factor.  M.2 disks come in two varieties - SATA and NVMe.  

A SATA M.2 disk will perform no different than a 2.5" SATA disk (all else being equal).  It's just smaller and doesn't require a drive bay, so it's convenient for very small builds and eliminating cable clutter.

An NVMe disk is substantially different.  It communicated directly over the PCIe bus, and is capable of much faster data transfer.  The only tradeoff is price, so if you can afford a NVMe disk, go for it.

If you can't afford a NVMe disk, then from a performance standpoint, the form factor doesn't matter.  Get whichever you like the looks of.