Can't really base this on much, as apple to apple comparison shows that no doubt BITMAIN's S2 looks to be a great box, but I can't help the feeling that re-using the 55nm chip is a bit "old tech re-factored". While one S2 will generally be more effective than equivalent GH/s of S1 AntMiners, the S2 just feels a bit pricey at the current prices due to the chosen approach and position in the newly changed market (Spondoolies "ultra cool" form factor and Chinese A1 copycat manufacturers supposedly selling 1TH/s at $3000 now).
When comparing physical density 1.25U (Spondoolies) vs. 4U (BITMAIN) the newcomer takes a clear lead, but it will be interesting to see BITMAIN's 28nm setup as it becomes available.
I'd be surprised if BITMAIN wouldn't drop the S2 price to position themselves more favourably.
Mature tech isn't neccesarily bad. For instance they've managed to provide a lot more chips for the same money and so are able to run them at low voltages to achieve similar W/GH as the next gen, but with the benefit of lower costs, ramped up production capacity and consistency.
Seems like lower cost is a major winner - unless you're paying for hosting costs and rack "U" space. Running something at home - I'd rather see less expensive and less noisy. If the Bitmain unit takes up a few more U in my rack - and is quiet and I can't hear it all over the house - AND it's cheaper - then that will be a major deciding factor.