Man there is already a difference to be honest. I have 16GB of RAM but my dad has 13" M2 Air with 8, and I've put it through the paces before and you can already feel how much swap it has to use. It's perfectly fine for him, he uses email and safari, but I push a bit harder and require a fan and have needed 16GB since years ago. I'm excited to upgrade to M4 Pro and get 24GB.
Apple Intelligence is going to speed up how different a machine with 8GB vs. 16GB feels for sure.
Also, if you do any gaming, the push into gaming that Apple is making is looking promising, and the machines with 16GB are going to do much better for that.
That being said, I think you are at a sweet spot to sell your current machine (or trade-in) and get either a 14" MBP M4, or a 13 or 15" Air (whatever your preference is) with M4 when that drops in a few months.
The longer you wait, the more your battery will age, the more your value will drop, vs. you doing this now and getting new design, new machine aka new battery and components, etc.
My vote is definitely take the value you are at now and get the M4 machine you'd prefer (14 MBP now or MBA soon).