Rather than go into detail, here's the big picture.
Better instructions per cycle (IPC), so similar clocks to SB will yield better performance.
Poorer thermal transfer, due to Intel manufacturing choices.
PCI-e 3.0 for future graphics cards, but little benefit with current generation.
More integrated controllers, specifically USB 3.0.
Lower, in general, pricing due to being "old."
Higher overclocks, in general, are possible.
"Only" PCI-e 2.0, but no real influence on the current generation of graphics cards.
Less integrated peripherals, generally leading to slightly higher motherboard prices.
If I were buying:
Pair a 2500k with a Z77 based board. In the future, you can switch to an IB processor, while getting a cheap and excellent overclocker now.
Additionally, read the Asus direct copper 2 review on this site. It may be worth getting, in lieu of what you have said. If it isn't, then at least it's something to consider.
You haven't seen anything until you've seen this.
Wow! I didn't know the blue screen of death could get a blue screen of death.