2.7GHz or 2.8GHz

They are the same, or the i5 would be infinitesimally faster, which would translate into no real world difference (certainly none above perceptual thresholds).

This article compared the i5 750 (2.66 GHz clockspeed) vs. the i7 860 (2.8 GHz clockspeed). Data shows the i7 inched ahead by an average of a 3% performance improvement on the whole. Make note that this is a lopsided comparison in favour of the i7 (if only marginally). The Macs you list would favour the heavier clockspeed on the i5. The i7 has hyperthreading (hence seeing the 8 cores) but that doesn't do enough to give it a substantial lead.

The article also shows the i5 outperforming the i7 in power consumption, which should be a deciding factor when buying a notebook.

But at the end of the day, you are comparing a notebook with a desktop, so it's a little lopsided. The iMac has a much better video card and a larger screen with a higher resolution. There is much more to a system than the CPU. On a whole, the iMac is a faster (better) machine, but not portable.

FMZ after looking at how you are using you system, aka with 10+ program open, in a development environment, you are going to more than likely have programs that can make use of all those cores, and even if the individual programs don't the sheer number of programs open will be better suited to a quad core. And if you come across flash content in a browser you will thank yourself for getting more cores in that case too, especially with a large number of programs open.

So since you are really going to be exercising your CPUs, taking a look at bench-marking results from Mac Geekbench results, etc will give you a good indication of how much total power you will have at hand. Going from dual core i7 to a quad core i7 is going to offer a noticeable improvement for you.

Oh and don't forget to get more RAM 8GB or more would suite you well.