Who makes a better healer?

[Arcwell]

Winona is technically better at strictly healing if you're just looking at player stats.

Though if you're going a team comp where you would think about having a Winona, a second Wilson is almost always a better option. Having a backup resurrector can make the difference between a win and a loss when your Wilson/Winona goes down to the Grand Forge Boarrior's line AoE attack.

Side note: I had a game once with Wilson/Winona/WX-78/WX-78/Wes/Wickerbottom. The Cooldown reduction from Winona combined with stuns from both WX's, Wickerbottom and Wes (he picked up Winona's hammer) made the Grand Forge Boarrior only able to attack once in between heals. We still lost.