You have 3 options really, stay at your current server and hope is a decent amount of people stay, follow everyone who changes every few days because they want number 1 for a few days or go to an established server where you will be way behind everyone however you know they won't be leaving.

At this point, other than group related stuff, the population of your own server doesn't really do much other than for chatting.
Fox can be finished in 5-10 minute after it stabilizes, regardless of population
Sage, Masuri, Arena are Xserver.

Given that, both staying in whatever server you are vested in and going to older server are mostly ok.
Staying is kinda boring, might have nothing to plunder but you might get more reward in GNW
joining older server mostly will make you a bit useless in GNW, get plundered instead of doing them and not rank as high in fox.
So frankly, I'd say the difference is not nearly as big as it once was, due to Xserver sage.
(note, if you are not on the oldest server cluster, moving to a different cluster might have some impact on game play. Thou unless you are near top of your cluster, it probably doesn't.)

There are two things you can think to do now that you can (in the sense that you can do before your server goes into the cross server circuit praiseluka was talking about): 1st reach sage of six paths rank in arena. 2nd get 1st as many times as possible in sage world battlefield. 3rd win 5 times every matsuri.
You have 60 days since the opening of the server to do it, at the 61st day you'll be in x server circuit for every single feature i talked about and this means for many months if not forever you won't be able to do it.
So, now that you can, do them. At that point doesn't really matter where you are. You'll eventually get merged with other servers later, so, for now, just do what you can by staying in your actual server. If you plan to continue to play for a long time this is the best thing you can do.
If not, if you want to chat with many people, create an account in an older server but you won't be able to be competitive there for months unless you pay a 5 digits quantity of money.