Assault, at low levels made me sad; though later, as i'm a fan of burst guns; the AN94 and M16A2 with x4 scopes and magnum ammo is bloody wonderful.
&&I tend to use the Smoke 'nade attachment; It makes Rush a hell of a lot easier and harasses the camping girlscouts (both on your team and the enemies). C:
And unlimited ammo for yourself, and the rest of your band of land-vikings.
Medic is always wonderful; if played right. Most medics seem to just rambo (not that I blame them; the M60 is beautifully brutal).
They may drop the occasional health pack; and a majority seldom revive you (particularly in Hardcore mode when choobs don't have a minimap or the cute little symbol).
It depends on what you really want to do; they both fill different niches in the game; I find Medics to be more useful overall, particulalry when playing Attackers; ten lives can often be the difference between glorious victory, or "lawlusuk u got pwnd"
Though leveling both fairly equally will allow you to be a team/squad player; you may find that you need to switch from kit to kit; depending on the balance of your team and the situation.
...or just pick whatever unloads the most metal in the largest amount of bodies and roll with that.
Personally, I tend to use Assault in normal modes and Engineer (PP2K <3) in hardcore; I'm not a real fan of Medics; more aesthetically than anything; the US Medic looks like a gay porn star, and the Russian one looks like a 'riod addict. And snipers, f*ck 'em.