Rich,
I'll weigh in with my thoughts only because I currently own and have used both the 305 and the 505 for some time now.
The 305 has provided me very good results with absolutely no problems ... so much so that, after 6 or 8 months of experience in various detecting conditions, I decided I would buy a 505 and keep my 305 (as a backup and to let friends use). I have been very pleased with my 505 and continue to be extremely happy with both units.
The additional settings of the 505 are well worth the additional $'s but now I wish I had jumped to the 705 for even more features. I can highly recommend either one (305 or 505) as an entry detector ... but if $'s is not an issue, consider going with the 705 for the additional settings and the additional features offered on the 705.
I wish I had automatic GB because my hearing is not so good (some high frequency hearing loss) and I have some difficulty differentiating between the high and low tones when groundbalancing. A good set of headphones helped me out with this but it would be a time saver to just hit autoGB, tracking and then go.
A backlight would be very nice in low light situations. You may not think it important if you will only hunt on sunny days or if you 'hunt by tone' but the backlight for use in the dense forests on very overcast, cloudy days would be nice.
If you have not done so, I would suggest you read Randy's ebook on the X-terras. In fact, if you haven't read it, then re-read it again and again, you will be missing out on a very valuable resouce [I must sound like Yogi Berra]. Every time I re-read his book from cover to cover, I learn something new, something I had overlooked, or something that he was telling me but that I couldn't grasp until my mind was ready to grasp it.
To sum up, you can't go wrong with either the 305 or 505, just expect to want to upgrade to the 705 in a short time.
Hope this helps.