I'd be more interested in the 8950/9950 for litecoin mining, the **70 might be better for bitcoin if you still want to use GPUs for that.

Currently the 7970 is bandwidth limited and despite having more processing power than than its little brother the hash rate doesn't scale.

The number quoted for the **70 show a higher band width, but with more processors you are still going to be bottle necked. If the **50 has the same band width but less processors and a much lower price then this is your LTC baby.

BTC you don't worry about memory performance and under clock not over clock. So the extra processors will be put to full work *IF* you want to mine with a GPU.