Shell Game Method
My first Heinlein was Stranger in a Strange Land, whereas my husband's first was Starship Troopers; I guess it just depends on taste and chance. As for being dated, I'd say his style might be dated in some instances, and his demeanor is idiosyncratic and somewhat aggressive, but it is interesting how, once you dive into Heinlein, you become used to it again and the style simply puts you in the "Heinlein frame." I wouldn't call his ideas dated, only perhaps his social framework presentation in some instances. I got a little weary of the whole Howard family/Lazarus Long thing, simply because it seemed to be an excuse for returning to a "clubhouse of the mind" at the end of some books, rather than ending the book properly. I haven't read Heinlein in earnest for some years (but did pick up 4 early books about 2 years ago and enjoyed them, although they were a little haphazard in places.) His ideas, however, are controversial no matter how old. Consider the quote," An armed society is a polite society. Manners are good when one may have to back up his acts with his life. " That would certainly put him in a definite political camp. But I think his thoughtfulness on the controversies puts him beyond the pat arguments that might pop into one's mind upon reading that. (Of course, the pat arguments are useful when one carries them to their logical conclusions, and that is, after all, why I read any speculative fiction.)
In answer to your question, I'd put the books you listed initially randomly under 3 shells, move them about, and pick one. That's kind of how I ended up reading Stranger in a Strange Land, and if you like the one you get, there's always more! If you don't, you can always quit or put the decision off til later!
Cheers!
Lyta