Rodger, Welcome to the X-TERRA forum.
Since Minelab added Multiple tone audio and the ability to use the 3 kHz coil, there are fewer reasons now than there were when we had the X-50 and X-70. However, there are differences. The X-705 has more "adjustment segments" for sensitivity and ground balance, allowing the user to estabilish a more precise setting for each. The 705 has 28 notch segments and the 505 has 19. That difference results in the 705 providing a more "refined" TID. The 705 offers Prospecting mode for those who might be nugget hunters. The 705 offers a backlight for those who have a hard time stopping when it gets dark. The 705 offers Auto ground balance. The 505 is manually adjusted. The 705 offers Tracking GB for those who want their detector to maintain the proper ground balance while working a site. The 705 offers Beach Tracking for those who hunt saltwater beaches. The 705 offers Tracking GB offset so you can offset the ground phase and maintain that offset "on the fly". The 705 allows the detector to establish the proper noise cancel channel Automatically. The 505 is manual. Both offer Pinpoint mode, but the 705 has the option of Sizing Pinpoint.
Honestly, you are the only one who can answer the question as to which one is best for you. Budget is always a consideration. If you don't prospect, beach hunt or need tracking, either one will do quite well. If you want the detector to make the setting choices for you (NC and GB) instead of you manually selecting them, or you want to detect with some sort of tracking, then know that the 705 is the only one that does it. Both are excellent detectors and meet the needs of many. Since you are new to the hobby, if you would tell us a bit about the part of the world you live in, and what types of "treasures" you are searching for, we might be able to provide more information. HH Randy