I bought them both as I wanted to make some specific comparisons. With the advent of the 505, there are fewer differences than there were when we had the X-50 and X-70. Coil freqs, threshold, tones, to name a few. You are correct in stating the 705 has more features. But frankly, I consider functionality more important than features. Some of the reasons I prefer my 705 over my 505 are the Offset Tracking Ground Balance, narrower notch segments, Auto NC, Auto GB, the capability to obtain more refined settings, Pinpoint Sizing and Prospecting mode. Minelab refers to the depth of the 505 as "enhanced" and the 705 as "maximized". Although I don't know if the difference can be attributed to hardware of software, I do know the 705 will hunt +/- 10% deeper under similar conditions. Here is a post I made comparing the two, just a few weeks ago comparing coils on each model.
http://www.findmall.com/read.php?55,1127217,1127217#msg-1127217
I suggest you look through the product brochures linked in our FAQFAQ section. Minelab clearly describes the functionality of each model, and is pretty clear that the 705 is considered their "gold machine" of the X-Terra models.
Is it worth the extra money? You are the only one who can determine that. If it is going to sit in your closet, gathering dust, then it is not. If you're not willing to put in the time and energy to learn how to use the additional functionality, it is not. Asking questions is a good way to get other people's opinion of how a detector might perform. But it don't mean squat on how it is going work for you. You are the determining factor in that equation. I will say without reservation, if you are willing to put forth the effort and the hours in the field, there is no doubt in my mind the 705 will serve your needs better than the 505, for the types of hunting you've indicated you want to do. The 505 is an amazing machine for coins and relics. But if you're hunting for nuggets, the 705 is a much closer match. HH Randy