I debated the 505 vs the 705, but for me it was a no-brainer because the 705 has a Ground Balance Tracking feature -- it automatically adjusts ground balance as you hunt -- which is quite important when ground mineralization is variable. All of the ground where I am is mineralized, but it also might vary from 3 to 30 at the same location I am searching! Otherwise, you'd have to keep stopping and rebalancing your machine as you hunt to keep it detecting optimally; the 705 does it much faster (automatically) and more accurately than the hunter can. Clearly your mileage may vary depending on where you hunt, but for me it was a necessary feature in a detector.
The other two main added features of the 705 are Beach Mode (which I have yet to use) to help deal with mineralization and the dry/wet sand difference, and Prospecting Mode, which is an all-metal mode with a variable iron-mask. Prospecting mode is the deepest-seeking mode, period, but you lose target ID and differing tones; it's great for relic hunting, not just looking for gold.
They're both great machines. Note that only the 70, 505, and 705 can use 3khz-frequency coils, if that matters to you. (I love the 3khz coils! Hit hard on silver.)