Depends on what sort of bioinformatics you will be doing and what sort of other resources you will have access to.

I have a very ordinary laptop (i5, 8GB RAM), but I do next to no hardcore bioinformatics on it. I have a 10 year old desktop with a Xeon E3 and 32GB RAM for running interactive R, jupyter and IGV sessions.

For eukaryotic sequencing type things (Definitely variant analysis or de-novo assembly, but also most likely RNAseq, ChIP-seq etc), few laptops are going to cut it either way, and you need access to a server - either your own server, or on a cluster, or in the cloud. If you are doing prokaryotic sequencing stuff, then you can probably get away with mostly using a local machine. The same probably applies if you are doing methods development, or lightweight systems biology simulations (the biggest simulations go all the way up to needing to run on national-level supercomputers).

With all that said, I tend to buy people in my lab a windows i7 desktop machine with 32GB of RAM and 1TB disk. This allows them to do most interactive things they will ever need to do. For non-interactive we use the institutional cluster. We also store all data on network locations, and local disk is only used for installing software and temporary files. Very few laptops will ever provide enough disk space to store full, raw, datasets from eukaryotic genomics.

With the advent of WSL on Windows, it makes very little difference whether you choose windows, mac or linux - pick which ever you like using the most, or fits your budget best. Mac is probably currently the fastest (with its M1 line of apple silicon hardware), but its way more expensive. Faster CPUs will make your life easier, but more RAM will make the difference between being able to do something and not being able to do it. Given the choice, choose more RAM over a faster CPU - I wouldn't buy anything with less than 16GB if its going to be your main system, and really you want to be looking for 32GB of RAM.