There may be a difference in SSD speeds that can be "measured" using disk speed test apps.
But... in real life... you'll never notice them.
The 512gb is worth having for the extra space. The "extra speed" is inconsequential.
It's all about what you want... what you can afford... and if you feel like spending the money.
I believe the 512gb size (at least from Apple) is "the best buy" right now.