How can I keep settings consistent between Macs?
There really isn't a one size fits all answer, but I can think of four ways to accomplish your task if you care to spend some time setting this up.



Macs are Unix, so simply check in your ~/Library/Preferences and /Library/Preferences into some sort of code control system. You can then do diffs and most plists are text or easily converted into text (using PlistBuddy) so you can isolate exactly what you have changed.

There are numerous tutorials available - Penn's wiki on Manipulating Plist files is particularly nice and directed to people with unix skills. 



There are plenty of Managed System software like Apple Remote Desktop or others like Casper if you wish to get software designed explicitly to manage user preferences.



Server accounts would be easy to set up for minimal cost if you have a spare machine (or virtual machine). Again - some work, but mobile accounts do work well for many people.



Lastly, don't overlook simply migrating the user account from the mac that is correct to the other mac periodically and just making a system where you are disciplined about how you record the times you change a setting.