The best thing that both sides can do is to turn down the rhetoric and get a good contract. A seniority number only has value if you have a decent contract to back it up. (Example US Airways)
Just to ponder a statistic. In an airline with 5,000 pilots, 1% of your seniority list is 50 numbers. If you consider the differences in careers between a guy who is 50, 100, 0r 150 numbers (1,2, or 3%) junior or senior to you, is there really that much difference? I know, I know, some guy gets furloughed and some guy doesn't, but over a 30 year career there isn't that much difference.
Pilots can fight to the death over that last 1% or 2% in seniority while failing to realize that a 5% or 10% increase in compensation brings much more value to you. Every seniority list fight comes down to a very few factors, especially for airlines as similar as UAL and CAL. All this talk about wide bodies and career expectations are probably not very helpful. Arbitrators are very smart and they won't be fooled by these simple arguments.
Your best bet is to keep the seniority list on the back burner while you get a good contract. If you work together on this, the seniority list will take care of itself. Good luck. (DAL guy who is happy we did as well as we did and only hopes you guys do better, a rising tide lifts all boats)