Early 2013 rMBP CPU

You can never have too much power .

Seriously though, I would get the 2.7GHz CPU for that little bit more grunt buts thats me, depends on how much CPU intensive stuff your going to be doing, if its just browsing the net, email, occasional PS then the 2.4ghz would be fine.

i guess it comes down to justifying the extra $600 for 0.3ghz difference?

It depends....

The cost to upgrade 100MHz is not worth it, but if you need the extra processing power for specific CPU tasks, get it. For almost every task you'll do, you won't notice a difference. I work with a lot of video encoding and have found that the upgraded processor saves me a few minutes on some tasks. If you're not doing something like that I would save the money...

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any system lag is due to Mountain Lion and software optimization. I've been running Mavericks for awhile now and have had no problem...