Both, as group support roles (as opposed to melee-DPS specialist Swashbucklers), should have some bonus to group healing beyond just the base CCW. Warchanter, as a front-line melee/support role, should really be more about passive damage ablation (ie HOTs) while they attack, while the caster/support role of Spellsinger is more in line for active AOE group healing.
I think both should get access to Heal in T5 or the L18 core (not capstone), as that's really the minimum for effective party healing in upper Heroic and into Epic, but Spellsinger could also get Heal Mass and maybe some cooldown reduction bonuses, while Warchanter could get some AOE Regenerate spell from the Druid spellbook and maybe some "bubble" or bubble/hot skills in the vein of Cocoon. Neither, of course, should be as effective a healer as a comparably-leveled Cleric, but the combination of a Cleric and Bard (who gives caster bonuses, mana regen, cooldown reduction, etc. to the Cleric) should be greater than two Clerics.
Whatever the support is, it needs to be more than what it is right now. Adding an aggregate +5 damage or whatever is really meaningless. The bonuses should be enough that people get really excited about having a Warchanter(/Spellsinger) join the party. There needs to be significant stacking bonuses that really make a palpable difference...enough that, say, accepting a Warchanter to your party will be more useful than just accepting another dedicated DPSer instead.
10/20/30% melee speed, 5/10/15% Dodge (ignoring cap), 30/40/50 stacking elemental resistance, etc. Not adding stuff people will already have enough of anyway (ie Fort), not adding stuff that wont do most players much good (ie AC, at least till its fixed), not adding stuff that really makes very little difference (ie +1 Attack or +1 Saves vs Fear), not adding stuff that's good but too little of it (ie 6% Doublestrike). Melee fighters should be able to feel like they can play much more aggressively with a WC alongside them (if they don't already play like zergs who think they're invincible...)
Same logic for Spellsingers: they should give palpable stacking bonuses to USP, spell cooldowns and animation speed, mana regen, etc...casters should be able to feel like they can play much more aggressively with a SS in the party.
And just case I didn't make the point succinctly enough...Bards should give sizeable speed bonuses to their party: attack speed (WC) and spell cooldown reductions (SS) plus movement speed (both)Adding an appropriately-specced Bard to the party should make any quest, any fight feel noticeably easier, or at least faster.