Arguments against either: Missing the kickstarter when wanting to go all in on a CMON game is bad news. Until a few weeks ago that would have meant that going for Massive Darkness would have been preferable, but I believe that the late pledge option has now closed.
Definitely something to consider. If you are a "completionist", it won't make you happy.
When looking at it objectively, however, I don't think it is such a big deal with these two games.
In the Z:BP Kickstarter they offered tons and tons of extra characters to the backers, many of them exclusive. But you simply do not need a hundred different characters to play. Really not. All the important expansions are available in retail (abominations, archers, crows). And there are plenty of characters to choose from still (the ten guest boxes alone make sure of that).
In the MD Kickstarter the excklusives have been mostly roaming monsters. There is a pretty significant number of those offering a lot of variety in that regard. But even there you will have plenty options in the retail expansions. The biggest "loss" is probably the Black Plague Crossover kit, which allows you to use the zombies and wolves from Z:BP in MD. Those offer a whole new type of opponents (undead).
In general, the exclusives in both campaigns are a lot like "more of the same" to the degree of being excessive amounts of "the same". I don't think the games will not work as well without them.
The Z:BP core box works just fine as it is. Some more heroes for a bit of choice are nice (and available with the hero box and guest boxes). I am sure it will be the same with MD.