Generation and model line matters more than small clock speed differences. The new MBA 11" is a much faster, much capable machine than the last generation one, but it is still not as fast as a MBP 13", and if the speed difference between the two MBA 11" models is really going to matter that much you probably want to get the MBP 13".

That said, it may be worth the $100; for the LV Sandy Bridge chips they use, the relevant difference is not the 1.6ghz to 1.8ghz jump in BASE clock speed but the 2.3ghz to 2.9ghz difference in the turbo clock speed.