yeah...not easy to make decisions especially when that much money is involved. The only piece of advice I can offer is if you can't try them out, then get the opinions of people who have the different systems. From what I have gleamed from the various discussions I have had, if price is a factor then the ikelite housing is probably the most economical ( I have actually gotten some quotes somewhere for nexus, sea & sea and ikelite).
As to the actualy use of the systems, I guess that depends a lot on what you want...each housing has its pros and cons (eg. size of housing, clear material to see if water is leaking (yikes!), moisture alarms, different viewing sizes, arms to fit, strobes that are compatible..list can go on and on)
Also do take into consideration what sort of lense you will be using as the cost of ports will be quite an issue.
My $0.02