Without knowing the speeds of each of the CPUs or the rest of the specs of the computer, I can't tell you.
The core 2 quad is a quad core but, more than likely (as I said you haven't listed specs of the computer or of the cpu), the core i5 will be faster per core. Whether you chose the faster dual core or the slower quad core, depends on what you plan to do with the computer.
so we need from you
-the specs on the CPUs
-the specs of the rest of the computer
-your intended use
Both #1 and #2 could also be solved by links to the computers.
Well the core 2 quad you are speaking about is q9000 i suppose. If so then the I5 is better in every single way (only probably the cache is lower) but even in clock speed so if you can afford it i suggest going for it.
Go with the i5s. They're newer and the technology in them is better and more advanced and it will be able to keep up with technology for a little bit longer (not that anything can keep up with the way technology is progressing. ).
depending on what you need, and you said it's a laptop, I think you should just get an i5-430m instead, because its faster per core, more energy efficient and I don't think you would need a laptop with 4 cores, you won't be able to use all the cores to the max because it's a laptop.