Parshos Maasei and Devarim both sport travelogues, of a sort.
Maasei details a stop-by-stop geographic itinerary from the very beginning of yitzias mitzrayim nall the way to the last stop—Abel Shittim in Moav. Rashi counts 42 stops in all, 28 of them in the 38 years of the gezeras meraglim, to explain that the portrayal of Hash-m’s kindness is the purpose if the travelogue and the raison d’etre of the Parsha, if not its name.
Contrast that with the quasi-travelogue detailed at the beginning of Devarim, where the Torah introduces Moshe Rabbeinu’s sefer-long mussar schmooze as an 9-stop shiur, naming places that Rashi points out didn’t even exist under the names delineated. Rather, they were euphemistic references to the commission of a certain monumental communal sin and/or its place of commission [e.g., “Di Zahav”, a nonexistent locale that refers to the chet ha-egel.]
Further highlighting the seeming contrast between the midas ha-rachamim Maasei travelog and the Midas HaDin Devarim travelog is what follows in each parsha: Maasei is a lot more positive, dealing with most of the laws that involve the bordering and governance of Eretz Yisrael; Devarim starts off dealing with why Bnei Yisrael were so delayed in getting there in the first place [see the retelling of the chet hameraglim in 1:22-2:16].
In a sense, one can view the difference in delivery styles: Maasei is G-d’s travelogue, Devarim is Moshe’s. And, theoretically, Sefer Bamidbar [and parshas Maasei] ends with what might be the ultimate inverse of the chet hameraglim: the desire of the benos tzelphcahd for their share in Eretz Yisrael; Sefer Devarim theoretically ends with either a] Moshe telling Bnei Yisrael that “Youre gonna mess up big time” at the end of Parshas Vayeilech, b] Hashem’s rather bloody revenge at the end of the Ha’azinu shira, or c] Moshe’s petira [and Rashi’s reference to “le’enei kol yisrael” as indicative of Moshe’s breaking of the luchos]. All in all—the Maasei travelog is a lot more positive than the Devarim travelog.
I would suggest that the approaches might stem simply from the very names of the sefarim, albeit in an almost counterintuituve manner. Much has been said about the dor hamidbar, specifically in this case how they were able to do [almost] nothing but learn for almost 40 years in the desert where the Eibishter really did provide. Everything. Hence, Bamidbar.
Devarim is a reality check of sorts, as the entrance in the land is about to commence and the direct provenance is about to come to an end. Moshe starts off the proceeding by saying, in effect, you guys has it so arguably easy for all this time, and see what kind of mistakes you made; what’s going to happen when you are not only responsible for a] your own provenance but b] each others behavior [becoming “arevin zeh lazeh” with the crossing of the Yarden? Boy, are you in trouble.
I would also suggest that the undercurrent of the Devarim versus Bamidbar approach is that life is a lot more like Devarim than Bamidbar, for two specific reasons. One, that real spiritual growth and challenge is less dependent on a 100% “Eibishter-will-provide” lifestyle [draw your own conclusions; and two, that despite appearances, it isn’t necessarily the better of the two; hence Moshe’s admonition of how things weren’t necessarily so great when they ostensibly were so good.
These are the best of times.