I've used the Excalibur for about seven years now and it's great. The iron mask is great for tuning out iron nails and spikes.
Upside?
-Finds deep targets.
-Finds really small gold.
-Can be set to ignore iron.
-Great tone ID on different metals.
Downside?
-Heavy, cumbersome and bulky.
-Freaks out around black sand.
-Quality of manufacture can be sketchy.
If you buy one set it up and submerge it in the bathtub for a while to make sure it's water tight. Why? My buddy bought on (on my recommendation) and we went down to Panama to look for Spanish treasure. The first time in the water his new excalibur flooded! He was so damned mad! I found lot's of great stuff and he was beside himself.
If it doesn't leak the first time it probably never will. I've been diving with mine for seven years and it's never flooded.
Coil size? The 800 has the 8" coil and the 1000 has the 10" coil. I had my 800 upgraded to a 10" coil but I wish I'd just kept the 8" coil. You can cover more area with the 10" but since you move pretty slow underwater anyway I think the 8" is better. Pinpointing is easier and you can get into rocky areas and ballast piles easier.
So that is my opinion.