I currently have a SanDisk X400 M.2 2280 128GB SSD in my laptop.
I'm looking to buy a Western Digital WDS240G2G0B M.2 2280 240GB.
I don't need a huge amount of space which is why I haven't invested more money, but I want to ensure that my system won't be slower as a result of the new drive?
Greens are better suited for laptops. you will not notice any difference in speed in real world use. Maybe in synthetic stress tests, but that doesnt matter. You can always connect an external drive for more storage further down the line if needed.
It will be compatible, it is a fraction slower but you won't notice any difference in everyday use. If you want the fastest you should be looking at Samsung Evo series 960, they cost more though