This is probably very inaccurate. I'm interested to see a comparison, these results vs actual results from players who have donated to your server. After 50-100 recorded transactions on your OT server, add this poll and measure the results then (only allow those who have donated to vote).
To see what the guys that actually fund your server wants.
I think they will go more toward pay to gain an advantage, if not unique OP items unaquired in-game through other means, then at least starting gear to get them started quickly.
Kito from mtibia measured his paying consumers, and found out that many of them are busy people. (earning a living). Thus small things that lets them enjoy the limited time they spend on the OT is important for them. You will loose some of these players if you don't have anything to compell them with.
You got to find the golden thread for your server.
Over the past several years I have noticed a similar trend as your friend but I also think there are reasons other than what you have suggested.
First off I would like to suggest that if a game is fun, and I mean it's really fun not just an addicting mind game like most, then people regardless of the amount of time they have to spend on the computer will enjoy it.
I also think that the cosmetic/utility option cannot be fully explored by normal OT servers due to client restrictions. A custom client with all new outfits/addons/mounts and even as Flatlander suggested shiny colors might be appealing. I think evidence of this is real tibia, they seem to be pumping out premium mounts and outfits like no tomorrow so it must be working. Another example is Path of Exile which is a free to play MMO-ARPG that deals almost exclusively in cosmetic micro-transactions (the exception being space in your bank to hold more items).
The big discussion is always pay to win and while I have to agree that most people seem to prefer an "advantage" or a "speed up" I think this is only as apparent as it is because once said advantage exists, it becomes a requirement to compete.
An example:
Zelda (Single-Player): You offer a 20% stat/exp boost.
League of Legends (PvP): You offer the same 20% stat/exp boost.
In these two scenarios I would be willing to bet that given any sample size, more players
proportionally on the pvp game will own said boost simply because it is now required for them to compete with the rest.
Now with all of that said, to answer your original question/interest. Synthetic and myself have actually been collecting data on every item purchased in our reward shop and it is very clear that items that provide some boost to gameplay are by far more popular (as expected on a pvp game). But one thing to note is that we run an 8.6 server that doesn't have many cosmetic options. Also another interesting thing that you might not even consider is random chance rewards (kinda like gambling). The most popular reward for us is a lottery ticket that lets you spin a wheel for random rewards. Now something to also note here is that each ticket is extremely cheap so that could also contribute to this statistic but the sheer number of tickets sold over the course of a year is astonishing when you compare other reward options on our server.
So a tldr for most that don't want to read an essay.
I think "power" donations are only popular because tibia does not have many new/unique cosmetic options. Also if said "power" donation exists then it becomes a requirement to compete in a PvP environment. The only way to truly test this would be to make a server with 2 worlds, 1 with power donations and the other with cosmetic/utility only and see which one is more popular.
If you really are interested to see if power donations matter I suggest you take a look at this games business model
https://www.pathofexile.com/shop this game has been around for years and has almost exclusively sold cosmetics, the only exception being stash (bank) space to store items.