Pat, if you have an EZ-Steer and T2 unit lying unused, you NEED to get that installed and tweaked before you do anything else. Once that is done, the accuracy of your line is 95% dependent on the accuracy of the GPS signals. As compared to manually driving or even following a lightbar, that other 5% is usually so small that it's hardly even worth mentioning.

I think the 500 has better mapping capabilities -- not a big issue for me.

I know the 500 refreshes the EZ-Steer @ 50Hz and the + refreshes @ 5Hz. EZ-Steer works with the 500 if you flash it so it knows about the faster refresh. I drool a bit when I think about 50Hz. It should make for a bit smoother guidance on curves but I do very little of that. It should also make for a bit smoother steering on straight lines. I've noticed that my + can waver a couple inches either side of where the GPS says the line is. Faster refresh may keep that number down a bit. Still, the big limiting factor is inaccuracies in the GPS signal and implement side draft. Most of the time (90%+) the wiggles in my lines can be attributed to one thing or another causing my implements to drift one way or another from exact center of the tractor.

But . . . and this is the big but: Last year and this upcoming year are and were low years for sunspot activity. That's gonna change and we'll need to change our list of "must haves" on guidance products. Here's mine:

1. Fast refresh, as that has the capability to do away with mechanical complexities and additional components used to determine steering "sense" that can be prone to failure; all of which can be replaced by simply adding processing power.
2. Access to both the US GPS and Russian GLONASS positioning satellites. The more birds you include in determining position, the lower the DOP is likely to be. Too many times during a typical day, the position and number of visible birds could be a lot better. Using both constellations only makes sense.
3. EU is also working on the Galileo system which will add another constellation. That one is dual freq. too.
4. Dual frequency receive capability has the potential to make the signals much more self-correcting in terms of propagation delay induced errors. This is here now, but seems to me that for what it takes, it costs more than I think it should. Hopefully that will change.
5. Improved satellite based corrections. Free dual freq. Canadian WAAS-like service certainly has potential. Hope it works out.
6. Doing some reading on the subject of GPS accuracy, I came across this:
• Finally, it is possible to determine positions to within a few tens of centimetres using a technique called Three Carrier Ambiguity Resolution (TCAR). TCAR eliminates inaccuracies by looking at the wavelike structure of the signal and determining how much it has been pushed along during its passage through the atmosphere. Such phase shifts can be measured to within a fraction of the wavelength, which is typically 20 centimetres. The calculations needed to do TCAR are presently beyond the capabilities of standard hand-held receivers and this technique is used mainly for long-term monitoring of movement in buildings, oil pipelines, changes in sea level and even changes in the shape of the Earth’s crust.
• As mentioned, the process seems to be calculation intensive, but CPU power is cheap and getting cheaper. I'd bet a Core2-Duo would be more than up to the task. What we're really needing is a self correcting signal direct from bird to receiver.  No one really wants to mess around with RTK towers and radio issues involved with transmitting correction info over, under, around, and through trees, hills, and buildings if they don't have to, do they?