The 605 offers the following with respect to auto-tunes and reminders:
1) Set a reminder for new
2) Set a reminder for all
3) Set a reminder for this
4) Automatically tune to this airing.
Both #1 and #2 will set a recurring, program-based reminder. This means that when you're watching TV, each time
an episode airs you will see the "!" sign appear in the top right-hand corner of the screen. Pressing ENTER will display the upcoming programs you've set reminders for and you can pick which channel you want to tune to. #3 does the same, but only once for the specified episode. Because it's now program-based (instead of time-based), you'll always get correct reminders. If the episode changes time, you'll be reminded at the new time. Or if it isn't new a particular week, you won't get any reminder at all. The 505 can't do this and is strictly time-based.
#4 will actually tune you to the program when it starts, which is useful for when you're recording it with an external device.
Because the system is now program-based (instead of time-based), recurring auto-tunes no longer make sense. A recurring auto-tune expects that you're using a VCR type device to record at a specific time (let's say 9:00pm on Tuesdays). However, the 605 is program-based meaning that rather than setting an auto-tune for 9:00pm on Tuesdays, you set an auto-tune for new episodes of "NCIS" (as an example). You've now got the 605 which is smart enough to know that NCIS sometimes gets bumped to a later time, or starts at 8:59pm, or isn't new at all some weeks. However, your VCR is still recording every Tuesday from 9-10pm. You end up with missed shows, incorrect recordings, etc.
By setting the auto-tune every two weeks, you always know when the show is airing and can adjust the VCR timers accordingly, ensuring you can record exactly what you want.
Other differences on the 505 versus 605 include smaller size, HDMI output, up to 14 days of Guide data (instead of 2.5), expanded programming information (NEW tags, etc), the ability to hide the titles of adult programs from your Guide, title search, and expanded Interests search. If your 505 is still meeting your needs, these features may not make much of a difference to you. As you mentioned, many of these mean a lot more when going from the 530 to the 630.