In my opinion, it depends on how you, personally, are most comfortable learning.

Jumping straight to 70, you're presented with a daunting array of skills. I could easily see this as being confusing to a player. It can be bewildering finding a skill that says, "May only be executed while under the effect of Dragonderp" or whatever, and have to search all the other skills and traits to try to figure out where that damn Dragonderp comes from.

Some are fine with that, but others might benefit from being issued the skills a few at a time. The job may play entirely differently at low levels than high - but the low levels are where you learn what the skills do on their own.

If you care at all about the story and lore of the game, the job quests often have some interesting things to tell. Thanks to New Game+, you can revisit them whether or not you use a leveling potion, but I personally feel a lot more satisfaction when the next job quest is something I can work toward.

And, let's be honest here - leveling in this game is super easy. You can reach max level in like a week or two of casual play (depending on what you consider to be "casual"). It's not like FFXI, where it could take the better part of a year to get a job to max level.

In general, I tend to think it's absurd to pay for a game, and then pay them AGAIN to play part of the game for you, but hey, it's your money, and you can spend it how you please!