As noted above, if you have a lot of decompression you need to consider lost deco gas contingencies and if the resulting deco on back gas potentially exceeds the reserve or the time gets too long and/or would inordinately delay the boat, you'll need two deco gasses. You can split a single gas between 2 AL 40s but if the deco is long, two different deco gasses will be more efficient. Again, you need to determine that you'll have enough of either deco gas and reserve back gas to finish the deco schedule in the event one is missing or unavailable.
Divers generally take a great deal of care to either keep deco gas on them (offshore), or place it in a cave in an area where it is not likely to go missing (farther back in the cavern, etc) even if it means having it on you or stashed well below the MOD as it's not unheard of for deco gas to go missing in some situations.
It's catastrophizing a bit by anticipating multiple failures, but in a lost back gas situation rock bottom needs to get you and a team mate to your first deco gas. However, what happens when the deco gas for one of the divers isn't there or isn't accessible as expected? In that regard, what you are comfortable with for a rock bottom could involve lost gas decompression planning as you need enough gas to finish the deco in the event someone does not have a deco gas. It also does not matter whose deco gas is missing or un usable at this point either. Obviously if your buddy was on your back gas getting there, he has none to decompress on if his deco gas is missing, which means he's using yours, and that means you better have enough back gas to finish the much longer back gas schedule. Conversely, if he has his gas and yours is not useable, life just got interesting if you don't have adequate back gas.
Being prudent is usually a good idea and large reserves tend to make a great deal of sense in overhead diving for a wide variety of reasons.
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I agree with the observation that buoyancy will not be noticed switching from an AL 40 to an AL 80, but size will.
It's a little late since you already made the purchase, but an AL 40s are preferable to an AL 80 on a single gas dives in my opinion. At your current level, the deco gas on any single dive is probably going to be small enough that doing deco off your back gas will get you out soon enough with an adequate back gas reserve. I that circumstance an AL 40 is just more convenient, and two of them will still get your through a longer trip with less bother underwater than a single 80, especially if you keep it with you through the entire dive.
However an AL 80 will also be useable later as a stage and for a travel/first deco gas on a two two deco gas dive, so it's never wasted money.
Another option if a larger tank is really needed for deco is an AL 72. It's a 6.9" diameter tank rather than 7.25" and while it does not seem like in on paper, the difference in diameter is significant in the water. They hold 72 cu ft compared to the 77 cu ft in an AL 80, so you're only losing 5 cu ft total for bail out or deco purposes, and you're only losing only 2 cu ft of penetration gas when used as a stage. They are popular as rebreather bailout in some areas of the country as they are a good compromise between volume and form factor and I use them for stages and travel gas.
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100% and 50% make sense in AN/DP class as you learn to manage two bottles and become familiar with multiple gas switches, but lose utility almost immediately once you start diving trimix as 50% makes little sense. Even prior to that, the difference in deco profiles on the average 25-30 minute 100'-150' dive are minimal whether you use 50%, 100% or both on a given dive. Consequently, the major reason for 50% tends to be to ease the back gas requirements in lost gas contingency planning at that level.
