I do not agree with your argument. I fall on the side of unethical. Many are conflating issues but at the core it's all about intentions. Going into a deal you know you are not going to keep is unethical. If you buy it and then determine you dislike it -- that's not what is being debated. OP wants to buy with the full intent of returning it. This is not only an abuse of the return policy, it drives up the costs to other purchasers who want to buy new. Consumer protection laws and return policies are not intended to protect those who just want to borrow a product for 30 days. In addition, believe it or not, the abuse can be costly. For example, LL Bean had to revoke their Life Time Guarantee because it was being abused. I fear that this type of bad conduct will jeopardize a liberal return policy that is really beneficial for those people who need to make returns for genuine reasons. I would advise the OP to do reserach prior to purchase. If Apple fails to deliver a product that meets those expectations, return it. The ethical thing to do would be to request Apple to provide a testing unit or go to store.
You're making a well-reasoned argument and I think there is general agreement to your point that abuse of these return policies could lead to them being taken away, as the LL Bean example you cite.
But I think that it is going a bit far to call this specific case bad conduct, or abuse, and here is why. The distinction here is that the buyer would be making a purchase knowing from the beginning that one of the machines would be returned. But not both, since the buyer does intend to own one of the candidate machines. The buyers problem is that he needs to determine which of two models is going to be satisfactory.
You suggest that the buyer should make the request to Apple for a 'testing unit' (something I don't think exists, but maybe I'm wrong), or go to a store (something that is at least temporarily not possible, but also for many buyers who don't live conveniently close to an Apple store this is not a very practical option. Not everyone lives in a large enough city to have an Apple Store, or within reasonable driving distance of one.)
So what does the buyer do? You advise the buyer to do research and make the best decision possible and order the model he thinks is most likely going to meet his needs. That is completely reasonable. If he receives it and decides it is not satisfactory, then he is free to return it, and presumably order the other unit that he has already decided would be his alternate choice.
The end result of that course of action is perhaps at worse a 50/50 change that the product will be returned, whereas ordering both at the same time means that there is a 100% chance that one of the products will be returned. The first is clearly preferable even though doing it that way is less to the benefit of the buyer because no direct comparison would be possible. Which brings up the point of how often do any of us have the opportunity to use multiple computers for a period of time before finalizing our choice?
Personally I do not expect to find myself in a position where I would be so undecided between one model vs another that I would consider doing something like this. That is where most of us probably find ourselves. The unethical person would do this as a general practice and without any second thoughts. The OP here is in my view in between these two. He does not sound like someone who does this kind of thing on a regular basis, but he is genuinely having a hard time determining which machine will be best for his needs, and recognizes that Apple does have a policy that allows him to do this in his particular situation, to help him make a buying decision in the comfort of his own home. I don't think that is unethical, but it is taking advantage of Apple's return policy with the full intent of using it to return the one he decides not to keep.
Question - back to your suggestion of asking Apple for a 'testing unit'. Suppose that the OP does that, contacts Apple, and explains his difficulty. Let's hypothesize that Apple responds by telling him to order both and return the one he doesn't want to keep, since they don't have 'testing' units, and that would be the next best thing. Would you consider that to be acceptable?