The rumors are calling for the 870/880 to be cheaper than their current generation counterparts. Their are also leaked 3D Mark screenshots showing the GM104 GTX 880 at 20% faster than the GK110 GTX 780 Ti.
Its usually a regrettable choice to purchase a hot, noisy, power guzzling last gen video card right before the new generation is released. In this case, you will at least have 4 months before you have the opportunity to regret your purchase.