My buddy got a Domino a few weeks ago and I got a Southard last week. So I really need more time for a totally intelligent view point, but here's what I think as of right now.
If I didn't already have a Sage 1, I probably would have bought a Domino instead without handling either. That being said, after handling both, the Southard just has a special something about it which is tough to tell from pictures or words. There is something about how it fits in your hand, the shape and the quality fit and finish that BEGS you to use it.
It doesn't flip as good as the Domino, yes. However, it still flips quite nicely and locks up rock solid.
The blade isn't as slim as the Domino, yes. However, the hollow grind, stone washed finish and shape are just great and I have yet to find a situation where I felt it lacked cutting prowess.
The choil on the Domino allows for better choking up on the knife, but in terms of full grip the Southard feels much better. You can still choke up on the Southard pretty easily and the extra blade length makes you feel like you aren't missing anything. In fact, you are gaining a lot.
You mention being unsure about the blade shape and spine, but I think when you get one in hand you will appreciate it much more. Sliding the thumb forward finds that little groove on the top and it cradles the thumb perfectly. The stone washed blade with that extra thickness just feels so good... it feels almost naughty. Seriously, I don't know what it is but this is the first knife that sliding my finding down the spine makes me feel a little guilty... like I'm cheating on my girlfriend or something.
So many Spyderco knives fall into that territory of "once you hold em, you appreciate them more" and I feel like the Southard is so right on the money in that regard. The blade to handle ratio, the ergos and the fit and finish just amaze me.
Really, there are two things wrong with it. The pocket clip is silly (sharp tip, too shiny, and personally I like deeper carry) and the brown G-10 is bland and too grippy. I've already ordered a replacement clip from STR that solves problem #1 and I plan to get some replacement scales to solve problem #2. After that, I see it being dangerously close to being my perfect knife.
Also, in no way construe this as being a knock on the Domino. I think the Domino is a GREAT knife and it manages to capture classic Spyderco styling cues in a perfect flipper design. However, it just doesn't have that special something the Southard does IMO. If they release a sprint version of the Domino with different scales, I will more than likely pick one up down the road.