hmm... interesting question, but a little information is missing.
As Sacred "also" offers multi player, is this meant for PvE or PvP?
Nearest guess would be PvE, but I like to nit-pick...^^
That's an important point. "once you get them up and running"... which probably isn't level 25 or so.
Reasons why I chose seraphim for both options are:
Bfg rocks from the beginning to the end. If the 3 buff built is chosen (Battle Stance, Bfg, Warding Energy), and if they're halfway properly skilled, they can be played mouse-only. Which hurts "fun to play". If the 2 buff built reliant on Divine Protection/Dashing Alacrity combo is used, then it's a bit more interesting. Not by much though, since recasting isn't that complicated.
The reason why I prefered seraphim over elf on the power scale was the elemental damage focus the elf needs to have in order to match (sometimes surpass) the seraphim. If (again) the enhancement mods for the Bfg were chosen, one mostly deals physical, working on all bosses. Crowds will not go down as quickly as with the elf, but the seraphim compensates with Pelting Strikes/Soul Hammer combo, one-hitting 3 opponents in close combat, which also reduces crowds to nothing very fast.