I definitely agree that N-gacha will give the most growth over time compared to N-Wilds. But the question comes down to whether or not you want to spread the growth over a few days/weeks or get an instant injection. Plus, you have to consider if you don't have 100 R Wilds on hand and get an R slayer. R slayers aren't worth NEARLY as much if you don't max it to 100 immediately.

The point I'm trying to make is rules like "never buy N-Wilds" are not necessarily good rules. There ARE times when you may need N wilds, and it may be worth your while to bump up 3 or 4 Moe cards to level 35 to get a 10k boost rather than get that boost over time. Or like I said, you may get an R slayer and realize, oh shit. I don't have any R wilds. Oddly enough, R wilds are somewhat harder to get. Sure, you can get two guaranteed R wilds every day easily enough, but events drop SR wilds (1/3/5) pretty regularly. When I look at my deck, I have SIGNIFICANTLY more SRs at level 100 than I do R cards. SR wilds are definitely WAY more precious, but they're also WAY more plentiful. Hell, after the last three events, I stockpiled the equivalent of about 800 SR wilds. By comparison, even with the two free ones per day, I only had about 200-300 R wilds.

Tell me if I'm wrong. Look at your deck. Odds are, you will have nearly as many R cards as SRs. At current, I have almost the same number of N/R/SR. All of my Ns are level 100. Half of my SRs are level 100. ALL of my Moe SRs are 35 or higher. If I add up the cumulative levels of all my R cards, I would say at most, I have 65% as many R levels as I do SR levels, and that's with R cards coming up in Chance Time MUCH more frequently.