No, the difference won't be devastating. In truth, while i5 is definately quicker and a new gen of graphics cards will (almost) always be faster I would be astonished if most users would ever really notice the difference unless they had both machines side-by-side. Frankly, I'm pretty convinced that the bottleneck in modern machines is no longer the processor or GPU (unless you're doing something that taxes both those areas obviously, gaming for example) but the hard drive. We're moving larger and larger data sets around and you can really feel the difference now between traditional HDD's and SSD's. But as a large capacity SSD still seems to be made of solid gold(and priced accordingly) that's a discussion for another day.
I've recently gone from a mid-2007 17" MBP (2.4Ghz Core 2 Duo, 4Gb, 160Gb 7,200rpm drive) to a current 2.8Ghz 15" machine with the same memory and the 500Gb 7,200rpm drive and frankly, it's not that much quicker in daily use. Oh sure, when I'm processing large numbers of photos I can feel the difference but the 17" machine wasn't exactly slow. And that, really, is my point. For the first time in my life (bearing in mind I've been around PC's since the 486 days) I didn't feel the need to upgrade because my old machine was slow. In terms of real-world performance in the last two and a half years I haven't ONCE been held up by the hardware and I really believe that for the vast majority of users current hardware levels are more than enough.
I think we've reached on odd point now where hardware continues to improve but there really aren't that many applications taking advantage of it. Gaming is an obvious exception to that, and if you're a pro user who regularly imports and works with LARGE batches of files where even a second difference per file will make a significant impact to your workflow then any boost is worthwhile. Otherwise just buy what you want and be happy with your purchase.