Choosing a new iMac

I want to purchase a new 21.5" iMac, mainly for internet use and photo editing (Photoshop Elements and Lightroom 4). In terms of performance and speed, I'm wondering which would be a better buy: 2.7 GHz with 16 GB memory, or 2.9 GHz with 8 GB. I really can't afford both the faster processor and the additional memory, and wonder which would be more beneficial. I plan to keep this as my main computer for 5 or 6 years. Thanks.

There's really no noticeable difference in speed as far as the user is concerned between the 2.7 GHz and 2.9 GHz CPUs. And, since the new iMac 21.5" machine can not be user upgraded with more memory, I would definitely recommend buying the 2.7 GHz with 16 GB of installed memory.