SP31s aren't too bad in a home setting if you underclock them. Underclocking is a good idea anyway because of the efficiency gains. You can get about 4 TH/s on 2.1 kW. Reducing the power consumption lets you drop the fan speeds substantially, which reduces the noise that they generate.
+1. at that sort of power level the SP3x is actually a reasonable volume and the efficiency/hashrate you get from a single box is far superior to the [relatively speaking] IT nightmare of running 4 Sp20 units at a similar spec.
What IT nightmare? Using a switch to separate Network connection?
Or are you talking about the need for 4 power supplies?
4x Sp20's running at 525Watts get about 1050GH/s +- 5% or less. Totals 4.2TH/s, with the ability to turn up the power for a total of an easy 6TH/s-7TH/s at below full blast.
I think they are both pretty good ventures if the space and budget allows. I like the density of the SP20 more though as it's easier to put in tight places or stack them without looking like a data center.
Either way, I am very impressed by Spondoolies miners vs. my Antminers before. The Interface, speed of system, options, easy of underclocking/volting, pool selection, etc..
ps.. I saw somewhere that someone recommended to remove the top metal for 'better cooling'. I suggest not. I guarantee it will increase your temps. The 'box' is designed to channel the high speed / pressure of the air for cooling. Making a huge gap on the top will totally negate this.