Minelab just recently announced a change to the Gold Mode algorithm for the 800. It was originally only going to run single frequency (20 or 40 khz) and based on field testing, Minelab has changed the default to Multi IQ (all 5 frequencies) with more weight/preference given to the higher frequency signals. Interestingly, this was the result of folks being able to capably find small gold using Park Mode 2 and Field Mode 2 which are also Multi IQ with emphasis towards the higher frequencies (though the Gold Mode Multi IQ algorithms 1 and 2 are better optimized for this than Park 2/Field 2 are now). Note also, that Multi IQ uses all 5 frequencies for both the 600 and 800 models, you just cannot select individual 20 and 40 khz frequencies in single frequency mode on the 600. Otherwise, the Park 2 and Field 2 modes are identical on both the 600 and 800, so you can find small gold with the 600, just not optimized as in Gold Modes 1 and 2.
Other 600/800 differences beside lack of the Gold Modes and lack of individually selecting 20 and 40 khz:
600 does not have personal profile save mode which allows you to save the current search profile with the press of single button (though you can save custom profiles in each of the two slots for each search mode). This is accomplished with a dedicated faceplate button which only exists on the 600 (minimizing the chances that you can eventually just SW upgrade the 600 to be identical with the 800).
600 does not have manual noise reduction mode selection only automatic.
600 does not come with Bluetooth AptX low latency headphones nor the WM08 wireless module that lets you make your favorite wired headphones wireless for the Equinox (this extra hardware alone probably makes up for most of the 600/800 price differential). Only lower-end wired heaphones provided with the 600.
600 does not allow adjustable non-ferrous tone breakpoints or non-ferrous tone pitch or volume adjustments and does not allow threshold pitch adjustments.
600 has fewer discrete recovery speed levels (3) vs. the 800 (8) and fewer Iron Bias levels (3) vs. the 800 (9).
Probably missing some other differences but the $250 premium for the 800 seems to definitely be worth it IMO.
The Minelab site discussion blog that has a more detailed description of Multi IQ and how it is implemented in the various Equinox detection modes is linked in this Tnet thread:
http://www.treasurenet.com/forums/equinox/565990-equinox-treasure-talk-3-a.html#post5665710