Milwaukee Packout.
Dewalt's new 2nd gen ToughSystem isn't out yet, and I'd be hesitant to buy the 1st gen in the meantime.
There have been complaints about Milwaukee's Packout roller box wheel failures, but replacement parts and I believe aftermarket replacements are available.
A job box is going to be easier to secure than individual boxes, but as you mentioned you lose out on portability.
You could mix and match, a job box for the bulk of things and a more mobile box - even one that stores inside on top - for tools or parts you really want mobile.
I guess it depends on how much you want to keep portable.
Only knowing some of what a millwright is typically tasked with, I'd think a tool box stack on a dolly or atop a rolling box would be the better way to go.
