Probably you will have to tinker with the ram settings inside your mobo to get it stable. IMO it comes down to you personally. Do you have a lot of programs running at the same time? Than go for the 24GB. But since it’s a planed upgrade you can benchmark your favourite games befor swapping, so you will see the difference yourself.
I try look for higher sold numbers and review amount. Than looking for the sellers rating and also numbers.
A lot of brands have official AliExpress shops which you can reach via their website.
On the other side, get yourself a limit you’re comfortable to “lose” and buy expensive stuff locally.