Sometimes a larger SSD may perform better because a larger SSD usually utilizes two or more NAND chips. As NAND memory chips become more dense, smaller SSDs may only require a single NAND chip. SSDs with multiple NAND chips are usually desired.
Details on the Apple Silicon Macs' SSDs are confusing and may be a moving target as many manufacturers tend to change how things are manufactured without any notice or model changes, so there are never any guarantees. Here are two articles which may help you figure things out regarding the SSDs used in Apple Silicon Macs:
https://arstechnica.com/gadgets/2023/01/entry-level-m2-pro-macbook-pros-have-slower-ssd-performance-than-m1-versions/
https://arstechnica.com/gadgets/2022/06/m2-macbook-pros-256gb-ssd-is-only-about-half-as-fast-as-the-m1-versions/
The links within those two articles may provide better & more clear details.
How much free space you need on an SSD greatly depends on the workloads. If lots of large files will be modified such as with video editing, then having more space on the SSD is best both so you don't run out of room, but also to help with performance and endurance of the SSD. If the SSD will mainly be used for storage or editing of smaller files, then less free space is needed. Just make sure the SSD is able to handle all of your current storage needs and any future storage needs since the SSDs in Mac are no longer replaceable or upgradeable after purchase. Apple Silicon Macs don't really have traditional SSDs as they just have NAND chips soldered to the Logic Board to work with the CPU.
Keep in mind memory is also not upgradeable after purchase, so make sure you pick a model with enough memory for your workloads.
Another consideration is the ports on the laptops as lower end models tend to have just one or two USB-C ports. Even the higher end & physically larger MBPros only have three USB-C ports now. Lower end models may only be able to support a more limited number of external displays.
