Well the Viking and Excal are based on the same body extrusion both the half milled and full milled bodies are available for each, along with the Diamond Dust finish.
The biggest and physical differences between them is the Excal is closed bolt, so has 2 solenoids and 2 rams to perform closed bolt operation.
This has a couple of advantages over the Viking, firstly since the bolt ram only has to close the bolt the cycling pressure can be run at a lower psi than the Viking 50-55 opposed to 60-65. This inturn lowers the over all kick of the marker which I personally believe is where people see the accuracy advantage.
The Excal is slightly more efficient since it does operate at it lower psi than the Viking (140-175 compared to 150-200), I expect you to see about roughly 100+ usable shots more from an Excal than a Viking.
The other thing you have to bear in mind with the Excal is that since it's closed bolt when you raise the BPS you feeder has got to be able feed that BPS, since the bolt has to move at the set BPS for every cycle. In other words if you set it to 18bps the bolt will cycle in 1/18th of a second.
The Viking since it's open bolt isn't quite so picky about loaders . . . but as with any open bolt gun if you set the BPS at 30bps you'll out shoot a Revvy easily!
The biggest difference is obviously the cost and a lot of the differences come down to personal preference. . .most poeple who have shot both often perfer the Excal.... also the Excal has a little bit more exclusivity which appeals to a lot of people!
Otherwise both are incredibly consistant and probably the easiest gun to maintain on the market.
Hope that helps!
Steve