*"Consensus is that even though Ivy is faster per clock, Sandy can typically overclock enough more to compensate for this"*
So in this case I could just buy the slightly cheaper 2500k then?
My cooling is Mugen 2 Rev B, I have one 120mm 500rpm fan but I'm willing to have 2fans for push pull but no more then 500rpm(thats my noise limit).
I'm confident my cooling in push pull 500rpm is enough for 1.2v vcore, should I invest in the 2500k then or the 3570k?
You could indeed go for the cheaper 2500K and expect to get the same or better performance. The downside to this is that the 2500K will use more power under load, and as others have mentioned above, you might be limited in your overclock by how much your motherboard can handle safely (or your power supply).
Some scenarios:
2500K @ 4.4ghz vs 3570K @ 4.4ghz
In this case, the IB CPU will draw less power (let's say 90w vs 110w) and perform slightly better, 0-5% depending on the app or game. Downside? It's a more expensive CPU.
2500K @ 4.7ghz vs 3570K @ 4.4ghz
The 2500K is now the better performer. However, it's probably drawing more like 150w instead of 110w. Advantages include better performance from a cheaper CPU. Downside is you need to have a motherboard and power supply that will be ok with the increased power draw. It will also dump more heat into your room (only really an issue if you're loading your CPU a lot). These probably won't be real issues as both CPUs are relatively efficient - it's not like you're drawing 300-600w like an overclocked AMD Bulldozer chip - but it's worth mentioning.
For what it's worth, I've taken my Ivy up to 4.8ghz under watercooling without delidding it (at a safe voltage too), but your mileage may vary. There have been several posts by users who are having difficulty getting even 4.4 stable. Several users have reported taking a 2500K up to 5.5ghz without extreme cooling, though to me the voltage looks unreasonably high for continuous use.