I think it depends if we're talking about pve or pvp, but in terms of pve I would have to say Knight in the long run, Paladin in early levels.
My reasoning behind it is the following:
- Knight's HP pool is by far the biggest of all vocations so it would be easier at high levels to hunt effectively, it would be a rough start considering you will have to build the knight's mana pool in order to be able to spam or chain spells effectively.
- Paladin's will reign supreme at early-mid level pve, they will get good hp, hence they would be able to venture in better hunting grounds and they would also have a really good amount of mana to keep up with the spells that mages learn.
I'm not really a pvp guy but, again, if all other conditions are equal then HP is the most important attribute, that's why I think in an endgame scenario Knight's would have the upper hand.
I would say that team hunting would become worthless or at the very least a pain, as now a Knight could have the title of Knight but it only tells you the stats they have (+Hp -Mana) but you don't know his true role inside a party, he may have trained Magic level and Distance so he's more of a High HP shooter rather than the expected front man. And that would affect other vocations too, players would have the freedom to make..Idk, a Shooter with high hp/low mp, a Tank with balanced hp/mp that relies on spells for damage/healing, a mage that at high levels acts as a tank thanks to the colossal amount of heals he can spam and the defensive gear and skills a Knight would have previously had, etc.
So in a server like this it would be chaotic to identify which path a given individual has decided to take regardless of his shown vocation. The best approach I can think of to get rid of this issue is to erase vocations as a whole, let people train whatever they want as you said but also everyone will advance in hp/mp at the same pace, that would bring another set of issues as everyone would be the same basically so no variety, no special builds = less fun.
Albion is a game I tried where vocations do not exist but roles do, and they play an important part in pve and pvp. Role and spells are defined by the gear you choose to equip. Gain experience with them and you will become more proficient (Giving slightly better damage/spell power every step you advance but only with that type of gear), so you can effectively change "vocations" on the fly which not only affect your abilities, it also adjusts your hp/mana accordingly.