The main differences are (1) mapping and (2) training modes.

If you use the computer primarily for training, and you want the ability to set up interval workouts, have pace alerts, etc., then the 500 isn't your best choice. But you can probably still find a 305 that does have that ability. If you do multiple sports and don't require a barometric altimeter, you could look at a 310XT or any number of computers from other manufacturers.

What distinguishes the 705 from all the other fitness computers is the mapping capability. If you like to ride in unfamiliar places; if you're a ride leader or a participant in a lot of group rides; or if you're like me and can get lost on a football field, then the peace of mind from the 705's maps is worth every penny.

When you say "longer rides," what kind of times are you talking about? If you're talking about overnights, you can recharge any Garmin with a USB battery pack. If it's just that your battery is worn out on your (did you mean 405?), it might be worth it to find out what it would cost for them to replace the battery in it.