... and consists of SK, (Bard or Enchanter), Shaman, Cleric, Zerker, and Wizard. Who would you guys go with and why?
Your grp is a bit of a mismatch as it stands as far as DPS so you could technically go either way.
SK, Shaman, Cleric, and Wizard can all do damage with spells. Which means that mana reiteration, twincast, dots which leech mana, mana regen, and crowd control would be useful for the group.
The shaman compliments the berzerker/Sk with some DPS perks and can already slow meaning the enchanters longer duration no hassle slow compared to the bard isn't an issue. Which means you could pick a bard without concerns in relation to slow.
You have both a shaman and a cleric and I'm assuming the cleric is the primary healer, which means the shaman would be focused more on patch healing and DPS with some debuffing mixxed in. Which means the shaman stands to benefit from the enchanter.
So I suppose the better question here would be what are some advantages of a bard vs an enchanter. What does a bard bring that the enchanter doesn't, and what does the enchanter bring that the bard doesn't.
Enchanter - Long duration buffs, requiring long durations between recasts. Can have two aura's up at once. Provides Mana Regen buffs, longer duration charms, longer duration mezzes, targeted AOE mez, PBAE mez, mez nukes, mez debuffs, chaotic mezzes (gives twincast), their synergy is geared towards casters, not melee. Won't be proccupied as long managing a crowd. Enchanters overall are heavily geared towards supporting caster groups, but can also be useful in melee groups, but fail to shine as brightly as a bard.
Bard - Very versatile able to take on many roles. As a higher level bard you'll have access to mezzes, slows, charms, debuffs, buffs, OVERHASTE, and your aura's provide improvements to accuracy, crit chance, triple attack, double attack, spell damage, etc. They have Selos, which is amazing for getting around. Especially since with a guild hall almost every location you could want to go is only a couple zones off from a port location. Fade - Unlike most classes bards have the ability to drop aggro several times in a much shorter duration. Bards get double invis with AA's. It's the only class I know that has a double invis built into their abilities (IVU and INVIS). Bard crowd control tends to be cumbersome in that they can often find themselves not providing any buffs to the group while they mez ads, leaving only their aura to buff the group. Their buffs don't last very long. Charm/Mez has a short duration attached to it, limited to single target, or PBAE. Bards themselves are oddball class in that no other class works like a bard. This makes playing one, or automating one a unique thing and I don't think anyone really has a perfect way to automate a bard.
With the current group I'd probably say bard, because the bard can boost the wizard, shaman, zerker, and SK. With that said, an Enchanter is also a viable option and can lock down mobs faster and more efficiently. In the end, it's about having fun.