"You can play red cards as Mountains and blue cards as Islands" is so obviously the best choice for the mana system - it removes mana screw and flood while keeping slightly tamer color screw, it eliminates card determinsm in a way that turns every single card into a decision (but not an overbearingly complex one! it's binary and one of the halves is super consistent) and eliminates the need for the ugly hack of sideboarding, it keeps open the possibilities of rewarding people for comitting to monocolor or almost monocolor with multi-pip cards or wantonly mixing the colors; and finally, it opens up cool design space with lands and nonlands organically transforming into each other.
the problem with the Duel Masters/WoW TCG/Lorcana approach of playing all cards as lands is that it's an absolute variance killer. in Magic, an ideal starting hand has, say, 3 lands and 4 spells that you can use early on to establish your game plan. but it's very easy to not get exactly that. you might have 4 lands, reducing your early options, or you might have 2, making lower casting costs even more valuable as a stopgap. and the 4 spells you have may not curve out perfectly. you might have a redundant 2-drop and no 3s, or you might have an expensive spell you can't use yet but will come in handy later. there's a lot of things that could go "wrong", which allows the game to create many different possible openings even in the same matchup.
if, on the other hand, all 7 of your starting cards can be either lands or spells at will, the odds of having that perfect opening go up significantly. the 8-drop I was hoping not to draw until later? it's a land now. the extra 2-drop I can't fit into my curve? land. the off-color spell I don't have the mana source to cast? oh you better believe it's a land. (which is why, for the record, I don't really buy that the system allows for much in the way of color screw.) and since I start with 7 options, not 4, it's much more likely that, somewhere in there, I've got the options I was hoping for. the exact cards may vary, but the basic dynamic of an opener is pretty similar across games, so I have to design my curve a lot less carefully, and matchups become redundant and repetitive a lot faster. this is actually one space where Hearthstone's system works better than Duel Masters: by not requiring cards
at all, it lets them give you a much smaller starting hand, which does go some way toward reintroducing variance. in my experience with Hearthstone it's not really enough, but your mileage may vary.
my point, though, is that I think this argument:
The interesting thing here is that having fewer choices is often barely noticible but having no spells is devastating; and that missing land drops is on it's own often barely noticible (not always, more noticible in faster formats) but being gated from higher-cost cards is devastating, especially if the cost is 3 or 4. When it comes to flood and screw variance, the middle ground exists, but it's not very noticible, and I would say the impace of middle ground variance from the number of lands is less than the impact of middle ground variance from costs and effects of spells drawn.
is asking the wrong question. I agree that the middle-ground impact is "barely noticeable", which is why Magic's mana system is always the first thing on the chopping block for any new TCG, but noticeable isn't the same as important. it's like how Dominion starts you off with a couple Estates in your deck: having some of the cards in your deck, at any given time, be functionally dead makes the flow of play significantly more dynamic. in fact, the latter kind of variance you mention (variance of card options available) is in no small part a product of the former: that card determinism thing (great term, by the way) is the only way to ensure that sometimes you just don't have good options, and as frustrating as that can be in the moment, I think it leads to more compelling experiences overall if sometimes you have to solve for a bad situation. the value of the land system isn't that you might not draw the right amount of lands. it's that you definitely draw less of everything else.