As a general rule, no expensive watch is an investment per se, and ALL (with a few minor and very expensive exceptions) will depreciate over time. Breitling and Cartier watches are probably on a similar "level" so will depreciate similarly.
In terms of sizes, the Navi World is 46mm in diameter, the current Montbrillant is 40mm and the current Navi 01 is 43mm. However, they wear smaller than their size suggests as a result of the design : they are quite fine watches as opposed to a tank-like diver for example. You may find this link useful as it gives the specs of all the Navis including sizes -
http://www.breitlingsource.com/models/navitimer.htmlTo be honest there's no substitute for trying a watch on as the size may seem like one thing on paper and something else entirely on the wrist, as (as you rightly said) case shape has a huge bearing.
From your initial post it doesn't sound like you are overly bothered by what's inside the watch (i.e. in-house or not), so that puts you in a good position of being able to choose entirely based on aesthetics. Try a few on and see what draws you the most.