As of patch 6.0:

All healers are equally gimped, tanks (other than DRK) can pretty much heal themselves and the party if they know what they're doing. You are 98% DPS and 2% healing now, but your healing kit is much larger than your DPS kit in case of any problems (that one DPS who doesn't know how to avoid mechanics). Still though, I hope you enjoy 2-1-1-1-1-1-1-1-1-1-1-1 as your gameplay because that's every healer now.

If you mean which of the 2 remaining healers are less gimped... it will probably be Sage?

WHM is pretty much weaker in all aspects, other than immediate GCD AoE Healing with Cure III. However, MP is still terrible. If you care about trying to support your allies as much as possible as a WHM (with healing or with DPS when required), I wouldn't recommend WHM. Your lily gauge as a WHM is a DPS loss to use (3 healing lilies + 1 Afflatus Misery is still less than 4 Glare IIIs, so you use it as sparingly as possible rather than use to build it up). WHM has the least free healing output available, but can have a very strong burst healing in a short window if they spend their entire MP bar for it. If you want to play an easy straightforward healer without requiring much brain power, I'd say to go with WHM.

SGE is a healer that's more technical than WHM. It doesn't have as much immediate upfront healing skills and relies more on mitigation and smaller amounts of healing to balance out the damage. Simply put, you have more micro control over your healing. It's a healer that has barriers using Eukrasia (GCD) to change your GCD actions, but relies on healing abilities to mitigate damage instead (oGCD). Your main healing gauge is addersgall - think SCH equivalent of aetherflow, except instead of charging 3 aetherflow per 60 seconds, you recharge 1 addersgall per 20 seconds, directly changing how often you should use addersgall in your healing playstyle. All healers should be DPSing, but SGE also takes this to another level with Kardia - as Kardia can heal one ally you designate every time you attack. The healing may seem weak, but combined with SGE's mitigative gameplay (Kerachole, Taurochole, Holos, Haima, Panhaima), A good sage can make Kardia slow down or reduce the amount of incoming damage overtime (Using Physis II, Soteria, Druochole, and Ixochole will enable you to fill the gaps). The main thing to note is that SGE is more or less an all-round healer with minimal downsides as long as you aren't the type to panic and heal every small damage that comes in your way. Of course, this wouldn't matter if your party knows how to dodge mechanics because you'll still be gimped DPS.

And speaking of the gimped part... SGE has a secondary resource - addersting. You can use addersting to use Toxikon I / Toxikon II (based on level). To use Toxikon, you have to let your Eukrasian shields break. Here's more cons to addersting:
- You take a DPS loss when using this during combat because you have to apply a GCD Eukrasian shield and the addersting skill Toxikon II is only equal to the equivalent of a Dosis III / slightly higher DPS in AoE compared to Dykrasia II. So you basically trade off 2 GCDs for 1 GCD worth of damage. The only time it isn't a DPS loss is during a transition or pre-pull basically.
- Toxikon I is a complete DPS loss unless you're fighting specifically 2 targets, and only 2 targets. Only then is this resource used. A.k.a. you're rarely using this from lv 81 and under.

As for the less gimped part... you have Pneuma at lv 90 (120 second GCD AoE attack + heal) and Phlegma at lv 26 (2 charge AoE attack) to use to feel slightly less like a gimped DPS. You have a decent AoE DPS rotation in dungeons by applying your shields ahead of time so they break during combat in dungeons to gain Toxicon stacks. Your single target rotation is still 2-1-1-1-1-1-1-1-1-1-1-1 though.

Simply put, the addersting gauge is used very sparingly in a fight. It's certainly better than nothing since you have ways to play better with it, but you're comparing to having [limited usefulness] to [nothing] as your gauge so... eh. It's not very fun to build an entire job gauge on a very small niche and expect people to have fun with it without giving more to do, but that's my opinion.

6.1 will have MP adjustments and "potential job adjustments" but we don't know the full extent of it. Just don't expect too much, if previous liveletters were any indication to how much the developers ignore the healing role's problems.