ffxi servers are like different cities in the same nation- they all have their own personalities, keep different hours, and of course pros and cons.
Big Server life? Of course most would think Asura, but I view that particular server as pretty much just a marketplace.
Are you a NA night owl? Bahamut might not work out for ya- my experience was the NA population on that server goes to bed a lot earlier than I was used to.
Don't want waits for BCNMs/Ambuscade, or fight over pop locations? maybe a smaller server is what you actually want. Just be ready to deal with an auction house that looks like a drug cartel's downtown Miami Bodega- bare shelves and you question how in the hell that business hasn't collapsed yet. Not to mention an even more closed-rank mentality in many LSs.
I know its hard to "know" what kind of gaming experience you want until given the chance to experience different feels, but if you have at least a concept of the kind of things you're after, you may be able to target a few servers, then reach out to ppl on those servers to just get an idea of the mood/vibe/etc.
Best of luck- I'm convinced these days that the multi-server concept where all are not simply mirror images of the others is a
good thing. It allows different personalities to all find a home in FFXI.