What is a profitable rig to mine with?

At the risk of stating the obvious... the hash rate on many networks is growing exponentially, not linearly. I did the math, and decided not to mine because of it.

Here's an example in Bitcoin (probably the worst choice)

The BFL BF0005G cost $274 + SH totals at around $300

If you could use Star-Trek shipping and teleport it to your office, you could start mining tonight when the difficulty was still at 31 mil. So lets map that out.

In Aug @ 31 mil diff, you would make 2.201 BTC and pay 0.036 BTC in power
In Sep @ 53 mil diff, you would make 1.295 BTC and pay 0.036 BTC in power
In Oct @ 90 mil diff, you would make 0.761 BTC and pay 0.036 BTC in power
In Nov @ 153 mil diff, you would make 0.448 BTC and pay 0.036 BTC in power
In Dec @ 261 mil diff, you would make 0.263 BTC and pay 0.036 BTC in power
In Jan @ 443 mil diff, you would make 0.155 BTC and pay 0.036 BTC in power
In Feb @ 754 mil diff, you would make 0.091 BTC and pay 0.036 BTC in power
In Mar @ 1.2 bill diff, you would make 0.053 BTC and pay 0.036 BTC in power
In Apr @ 2.1 bil diff, you would make 0.031 BTC and pay 0.036 BTC in power
{stop here, out of power}

Looks good, If you start tonight with the BFL you will make about $200, BUT, if for some unimaginable reason, you don't get the BFL till next week your profits will drop to about $120, or if it is delayed 2 weeks, you drop to about $75, and if it is delayed 3 weeks, down to $27, beyond that it is a loss.

So in the above example, it is really a product of "how fast can they ship you the rig".

Don't know about asicminer, but BFL was delayed 8-12 weeks last I saw, and had a terrible reputation for delay.