well.. to be fair. Alumite is better than steel.
assuming all heads are compared with slime rods and feather fletching, we find that alumite is lighter than steel for the same base damage. 1.40 vs 1.98
Many otoh is much heavier for another half heart of base damage. 1.4 vs 2.59.
cactus arrows mass in at a very good 0.62
These numbers have a significant impact on the parabolic trajectory of any arrow shot, and hence has a big impact on ease of aim, as well as final range of the arrow.
any decent shot, uses arrows at medium to long range. hence the desire for lighter arrows that fly straighter. they are simply much easier to hit with, as well as the fact that long range shooting means loosing arrows. the expense of maxing out the alpha strike capability is not usually worth it. there is plenty of time to bring even tough opponents down to size by hitting them multiple times at these ranges. Heck. i routinely snipe skeletons from outside their aggro range. getting the sniper achievement is not a hard thing to do.
Now here is an important point to make: you must be consistent with your arrows. if you are constantly swapping out what kind of arrow you use, your aim at long ranges will suffer. even a small change in the mass of the arrow affects how much you have to deflect your shots in order to hit a target at any given range. this means that you have to constantly practice your aim with the arrows you are currently going to use in battle. There is therefore a hidden player skill penalty cost associated with swapping type during gameplay. later tier arrows have to therefore give a major long term advantage to be worth it. they might be better in PVP, but against mobs?
Here's where we get into Math.
To be able to determine if the additional half heart from Alumite is worth the decrease in trajectory, we have to determine how much damage you need to deal to kill opponents. Much like D&D and other games, an opponent is fully functional and just as big a threat at 'nearly dead' as he is at 'full health', therefore the only real considering factor in your damage output is 'how many hits it will take to kill an opponent'.
For example, if your opponent has 10 health, doing 5 damage is better than doing 4.5 damage, because you go from needing three hits to needing two. However, going from 5 to 5.5 is completely worthless, because you're still needing two hits to kill your opponent.
Most of the opponents you will be shooting at have 10 hearts of health. Spiders only have 8, and Endermen teleport when you shoot at them, so there's no point. Cave Spiders have 12, but generally they jump at you from around corners in the abandoned mines.
Therefore, the 'breaking point' numbers are: 10 (one-hit), 5 (2-hit), 3.5 (3 hit), 2.5 (4 hit), and 2 (5 hit).
For purposes of this discussion, we're primarily talking about either 10 (1 hit) or 5 (2 hit) point breaks, and how easy it is to get there.
For an arrow with a cactus head, slime rod, and feather fletching, the weapon does a base damage of 2.5, with a total damage of 5 at full draw. Right off the bat, it deals enough damage to two-hit most opponents you will be shooting at. Therefore, Alumite is a complete waste of resources and trajectory smoothness, without modifiers.
Now then, Nether Quartz for sharpness will modify an arrow to do more damage. If you can manage to get the arrows to stack (still a bit buggy, last I checked), you can put it on a whole stack at a time. If you are willing to dump 72 nether quartz on a stack of arrows (which might get lost or fall into lava or whatever), you can attain a theoretical 10 damage with Alumite versus 9 damage with Cactus with a single enchant slot.
Practically, of course, this means you simply need a single piece of nether quartz for the whole stack to match that damage output with a cactus head, which isn't going to break the bank if you aren't hesitating to put 72 on there already.
At this point... what other enchants do you really want on your arrows? Sure, you've technically blown two enchantment slots as opposed to one on the cactus head, but practically, it is just a single piece of nether quartz extra. You are now one-shot killing practically every mob in the game that you are going to be drawing down on.
So really, the question to ask here for yourself: Is it really worth having to cast all that alumite, plus the reduction in trajectory, for a single piece of nether quartz per stack of arrows? Personally, I don't think so.
But what about the EnderDragon or the Wither? Won't that extra damage add up after a while? The answer is: Probably, but particularly with the EnderDragon, you're going to want that extra range and smooth trajectory to hit the bastard accurately in the first place (not to mention getting rid of the towers). The lighter weight afforded by the cactus will translate into a higher accuracy rate at longer distances, meaning you will end up killing him dead sooner, and probably with fewer shots overall.
As far as the wither, he won't really get far enough, so you might as well go with Manyllium for the extra damage output afforded by it for the first half of the fight. The second half, of course, you have to go melee.