I'd be more worried about having people to actually group with while leveling. During Beta Phase 3, I logged onto one of my old 1.0 characters on Hyperion and there was NO ONE (that's not exaggerating, literally no one) in Ul'Dah or anywhere near.
A large number of legacy players could have rolled on new servers for the beta to play the new lowbie content and possibly recruit new players to their servers. Newer servers will probably have higher populations, but I think that there will be plenty of players on the legacy servers considering that there are fewer of them. I would wait until the open beta and then see how things are looking. I'm sure there are plenty of 1.0 players that didn't bother with closed beta and will be hopping on to see how the game is.
Also, I'm sure the population on legacy servers will be more stable than on fresh servers. There will be a huge influx of new players and then a drop off after a couple months, regardless of how well the game does. Legacy servers probably won't feel this quite as much as the new servers will. A lot of the new players will be friends of legacy players and less likely to drop the game early on.
Personally I'm rolling on a fresh server. I was unable to play 1.0 until a month or so after servers went down and I've never played an MMO at its launch. I would love to play with the high concentration of committed legacy players on the legacy servers, but I want to experience what it is like being there from the beginning on a server. That coupled with all of the exploits and bugs that allowed a portion of legacy players to quickly level all jobs and amass max gil with minimal effort makes playing on a fresh server a no-brainer.
I know a lot of people are saying that the economy will be balanced on legacy servers, but I don't think that it will be, not for a while at least. Even with a 10% reduction in the amount of gil everyone possesses, that still leaves plenty of gil to those that would potentially corner the market on certain goods. Sure you might not have to pay 100x the price of some lowbie item, but you would still have to pay more than it's "value". In WoW I've seen players with large amounts of Gold be able to dictate the prices of items to insane extents.
I guess I could be wrong to some extent and underestimating the amount of players that will be in the position to input items into the economy. I suppose that with a large amount of players having every craft capped it would be pretty easy to find LS/FC friends that could make you gear for reasonable prices.The choice could simply boil down to something such as that, and for a lot of people I'm sure it does. I know if I had any friends on legacy I probably would have rolled there.