Let's see some illustrative calculations:

Let's assume bASIC 57GH/s uses 120 W.
The cost of electricity (say 0.11 $/kWh) is about $58 per year higher than that of BFL SC Single.
If one buy this instead of BFL SC Single, the price difference ($230) pays for the more power for 4 years.

Let's assume bASIC 57GH/s uses 180 W.
The cost of electricity (say 0.11 $/kWh) is about $115 per year higher than that of BFL SC Single.
If one buy this instead of BFL SC Single, the price difference ($230) pays for the more power for 2 years

Let's assume the difficulty is 10 times greater than now (10 x 3054627), one block gives 25 BTC, and 1 BTC is $12.61.
In unlikely case of being them constant for a year, the Single net annual profit is 4488 $/year and bASIC net annual profit is 4261 $/year.
The difference is... $227. Interesting coincidence. (I didn't set the BTC price, it was left in my BTC calculator from two weeks ago.)
For 100 times greater difficulty the yearly net profits difference is $396 - $374 = $22.

I believe the differences are way below an error coming from uncertainty (of tech specs, of difficulty changes, of price changes, etc.).
The date when the mining starts is surely the most important factor.

EDIT: The 57 GH/s number should be 54 GH/s. The differences of annual profit are actually two times greater: $454 and $44.
The annual profit I'd calculated as if both devices use the same power (60 W for both), so I shouldn't call it "net".