Depends on how much you need what few features Z87 offers over Z77. Performance-wise, in most cases the 4670K is roughly 6% faster than the 3570K, but in most cases the 3570K will also overclock more than 6% higher than the 4670K while consuming less power under load. That said, you shouldn't really base your purchase decision between IVB and Haswell on overclocking, since it is a bit of a gamble and you may get a lucky 4670K that'll do 4.8GHz and you may get an unlucky IVB that won't pass 4.4GHz. The worst case I've heard so far is a guy who got a 4670K that couldn't pass 4GHz, which is only 200MHz over the stock turbo clock.
Even so, a 4670K at around 4.4GHz is comparable to a 4.7GHz 3570K, so the difference between the two will be minute, even if the 3570K can reach a higher clock. If the prices of a Z87 and 4670K are within $10-20, I'd say sure grab the 4670K. Otherwise there's not much reason to spend the extra bucks on Haswell.
But yeah - if you already have an LGA 1155 board and you aren't scrapping it along with your i3, you could save a bunch of money and just pop in an i5 3570 or something and nearly double your performance. Though if your board is B75 or H77 you wouldn't be able to overclock, so no point grabbing the unlocked 3570K.