Inspiration versus Surge of Light was one of my least certain decisions when I was putting together a talent spec. When I swapped to Smite from non-Smite, it seemed like Surge would be better given that Smite does not proc Inspiration but does proc Surge.
Well, I have some real data now, so I can actually see how much physical damage was dealt to people in the 15 seconds following me landing critical heals on them. I'm working from our World of Logs reports, so it's a little cumbersome, but foolishly I haven't been running my own logs up to this point.
The result for Magmaw is that Inspiration would have prevented around 90k damage, and on Omnitron it would have prevented 139k. On both encounters I healed around the same amount, approximately 3.15M. So as a percentage of total healing, it was 2.85% on Magmaw and about 4.19% on Omnitron.
I calculated the expected number of Surge procs given the coming patch that will allow Flash Heal and Greater Heal to proc it. Surge did not fare to well, with only 3.5 procs on Magmaw and 2.9 on Omnitron. Even allowing that the free Flash Heal will be able to crit *and* will be substantially more likely to benefit from Grace, this is substantially less than the damage reduced by Inspiration.
Of course these numbers are pretty far from telling the whole story, given the very different ways that the two talents function. Surge gives a heal that takes my time to cast but that I get to personally direct. Inspiration is sort of a blanket over the raid, randomly reducing bits of damage. But I think in the intangibles category, Inspiration has a bit of an edge for a two reasons.
First, Inspiration does not take time to cast to extra "healing".
Second, while I would normally be in favour of benefits that I get to personally direct rather than benefits that are handed out at random, when the personally directed benefits are so heavily dependent on procs, they become very random. I can't make the best use of an instant cast, free Flash Heal if I can only use it three times a fight and if my windows to use it are only 10 seconds long. If those windows don't correspond to the best time to use it, then I am having to make decisions to direct it without getting the benefit of making decisions. In this circumstance, I'd rather have it just be handed out at random.
If the way in which the benefit is handed out is preferable and it's a bigger quantity of health over the fight then the choice is pretty clear. I'll be moving those points over in my build.