Well, I have to admit that I'm currently waiting for the S4 specs and price be revealed. Although there is no valid point in investing in mining now with difficulty going through the roof (network hash rate has already spiked to above 260 PH/s, currently sitting at 250.5 PH/s) and BTC price in the cellar, I'm prepared to purchase my very first mining gear, because winter is coming and my plan is to let the miners assist in heating the house.
SP-T has really something going here, but unfortunately the long delivery times, high price and especially the high shipping costs speak against the purchase of a SP20 or SP35. Considering the total purchase price Bitmaintech offers a better $/GH rate, and although SP-T had a better W/GH rate so far, it may be beaten by a S4 with underclocked/undervolted ASICS.
$/GH and W/GH both have to be right, which is why those Tubes with W/GH way off can not even be considered.
I'm not giving up on SP-T, but something has to happen, because it's just not working that way. We need some innovation to bring W/GH below 0.5 and price + delivery times down, which may be impossible right now, but I have hopes for 2015.
Let's assume a static network hash rate of 300 Ph/s, power costs of 0.1 USD/kWh and an exchange price of 500 USD/BTC.
SP20 costs 2.64 USD/day,
SP35 costs 8.4 USD/day,
S3+ B9 costs 0.86 USD/day, and
AM Tube costs 2.16 USD/day.
SP20 at 1700 Gh/s would mine 0.0204 BTC/day or 10.2 USD/day,
SP35 at 6000 Gh/s would mine 0.072 BTC/day or 36 USD/day,
S3+ B9 at 453 Gh/s would mine 0.005436 BTC/day or 2.72 USD/day, and
AM Tube at 800 Gh/s would mine 0.0096 BTC/day or 4.8 USD/day.
SP20 would make 7.56 USD/day profit,
SP35 would make 27.6 USD/day profit,
S3+ B9 would make 1.858 USD/day profit, and
AM Tube would make 2.64 USD/day profit.
SP20 costs 1,500 USD inc. PSU + shipping, it would take 199 days to break even,
SP35 costs 4,035 USD inc. PSU + shipping, it would take 147 days to break even,
S3+ B9 costs 320 USD inc. PSU + shipping, it would take 173 days to break even, and
AM Tube costs 500 USD inc. PSU + shipping, it would take 190 days to break even.
The network isn't static though and AM Tubes are the only miners in stock. I'll leave you to draw your own conclusions.