I just ordered a 7970 from Newegg which is yet to arrive. Going to find out myself, but I was in the same situation you were when I first was looking into the mining.
With the block halving approaching in December of this year, I would say the 7970 is the way to go. The BFL singles take 2 months before they ship due to slow manufacture and high demand, and the 60 days could be crucial time that is needed to break even before the half.
However, your electricity rate may simply be too high to go for GPUs. In that case, definitely go for the BFL single.
In most cases, if your electricity rate is < $0.15 you should be good to go. Electricity can be a huge factor in the profitability. Just go look at the mining profitability calculators and plug in the numbers. It's EXPONENTIAL.
It's best to plug in at least 2 7970 or in conjunction with another card. That would give a more efficient MHs/w than just a single 7970.
If you have solar panels, by all means mine away with the 7970. $0.00/kWh beats everything