I just started mining 2 weeks ago. I now have 3 antminer s4's and I recently purchased an SP20 which will bring me up to 7.7TH's. If you really have free electric(I pay $0.095/Kw), you should be shopping for the best bang for your buck (GHs/$).
https://en.bitcoin.it/wiki/Mining_Hardware_ComparisonAlthough it's not the most up to date it shows a quick overview of most of the miners available. I tend to look at GHs/Watt, but you should really focus on GHs/$$, but also pay attention to how many watts the device uses per PSU because you'll still need proper electrical wiring and circuit breakers to support these ASIC beasts. For example if you only have a 15amp breaker you'll only be running 1 S4 or SP20 because they easily pool 1300-1400W on a breaker rated for 1650W(110V). As you begin to expand you'll start to realize you'll have to upgrade your electrical wiring to accomodate ASICs in a residential environment.
Antminer S4 ($1200 + $160 shipping) = 2000GH/s/$1360 = 1.47GH/s/$
Spondoolies SP20 ($695 + $119 shipping + $150 1300W PSU) = 1700TH/s/$964 = 1.76GH/s/$
(cyber monday price still available for SP20...$695)
If you can talk someone into giving you a $400 Antminer S4 coupon(Expires 12/15) then the pricing shifts back in favor of the S4 like so...
Antminer S4 ($1200 - $400 + $160 shipping) = 2000GH/s/$960 = 2.08GH/s/$
As far as usability the ASICs are very easy to setup. The S4 requires you to only have to plug in the power cable (14AWG gauge recommended and not included) and a network cable and then remote into the unit from your home network, setup your pool settings and your off and running in minutes.
I've not used the SP20 yet, but I assume its similar, except that I will have to plug an external PSU to the SP20 to give it power. I bought a Rosewill Lightning-1300 1300W 80 PLUS Gold PSU from Newegg for $182 to power the SP20. (P.S. you can pay with bitcoin on Newegg now!)
Hope this helps.