There is no doubt that both the core i5 and core i7 represent significant values. However, these are SYNTHETIC benchmarks that treat hyperthreading cores the same as the physical cores in the core i7 cpu. Thus, the 35% boost over the core i5 is not surprising. One should be aware, however, that in most real life situations, the additional computing power provided by hyperthreading is often only marginal, at best 5-7% beyond what can be achieved by the four physical cores, and this requires programs that operate in a highly parallel fashion, such as video encoding. For many applications, there is simply no difference and for some, there is actually degradation because the instructions provided by the application cannot be executed in parallel and result in the stalling of the physical cores. This is not a problem in PCs because hyperthreading can be turned off in the BIOS if one finds this effect or if the hyperthreading is causing the chip to overheat, which does happen. From what I have read so far, it would appear that there may not be any way to accomplish this in the iMac. Thus, one should consider carefully the type of software to be run before deciding. Bigger is not always better.
Also, do not be confused by Intel's naming protocols. The 2.8 GHZ core i7 on offer is NOT a Nehelem chip, but rather uses the same Lynnfield architecture of the 2.6 core i5 chip. Both use dual channel memory controllers (as opposed to the faster tri-channel memory for Nehelem chips), are capable of only half the video bandwidth of the Nehelem chip and also utilizes a higher latency, slower interconnect bus than the Nehelem core i7's. On the plus side, the energy efficiency of these Lynnfield chips is much better than the first generation Nehelem chips. They use much less electricity both on idle and at load than the Nehelem chips, which is probably why Apple waited for them to be released. Nevertheless, the 2.8 core i7 chip with hyperthreading will run significantly hotter and draw mopre power than the core i5. So if heat and/or power consumption are a concern, stick with the i5.
https://www.macrumors.com/2009/11/14/core-i7-based-27-imac-benchmarks-show-significant-improvements/
