I have the 500 now and had the 705 previously.

Garmin does have Garmin Connect which is online. They also have Garmin Training Center for PC's. There are however other programs that you can use. One of the more popular ones is Sport Tracks by Zone 5. So tracking data is not an issue.

Accuracy in my experience is not an issue either. Both the 500 and 705 will have accurate speed and distance measurements provided you have the speed/cadence sensor. Otherwise you will get the same speed spikes.

The 500 screen is not a problem for me. I have read the some posts by people that had issues but they were very few.

The 500 is still early in its firmware life cycle. Garmin is working out some of the bugs in it and people are getting corrupt data files at this time. Garmin has a good track record of getting it right but can take some time. The 705 firmware is fairly stable at this time due to the age of the unit.

It really comes down to what you really want it to do. The 705 has two main differences over the 500. Detailed street maps (if you buy them) and a larger screen. Garmin units are notorios for bad calorie calculations unless you have a power meter.