Since Alternative takes quite a bit time investment to 'get other endings' I wanted to at least address what changes by picking each girl, maybe it will be helpful to someone who doesn't have time to spend hours of autoskip for minor changes.

Obviously be warned there are minor spoilers so I recommend reading details only after beating 1 ending with your favorite girl picks.
Be aware there isn't even a single CG you will miss by going for ANY ending. So all you are missing is simple text. There is very little even on that front.
The unskippable animated scenes are what will take a very big part of your time

My overall recommendation after struggling through all 5 girls for hours is just to read my summary if you value your time - I give all changes below but the only somewhat meaningful scene change is at the very end which has youtube links for each girl below. Very little is changed. I do however recommend selecting the favorite girl since well.. you would want to see more of here scenes, no?
There is no 'best' choice, BUT if you plan to play only once and unless you have high preference for someone else - I think 'Meiya' is the most 'natural' route, all other feel a bit more clunky with how the main story plays out.
Differences are almost as small as in unlimited if not less, more text changes but no extra CGs/big scenes.
The reason for Meiya being my recommended pick is due to how main story is heavily scripted around having a lot of her scenes regardless of your choice, even super important ones. So when you don't pick her you are kinda... still semi stuck with her in most touching scenes closer to finale so it would be the most impactful.

SPOILERS START HERE!

The 'girl ending' changes happen starting with chapter 7, where you have to select one of 5 girls in chapter 7.
If you didn't make a save file there - after you unlock chapter select - you can go directly to chapter 7 and select 4th option, set autoskip and you eventually reach the choice.

I won't summarize what each 'heart to heart chat' contains but since characters are so one dimensional - you can easily predict what they would be like by seeing one girl scene and thinking what would the other girl say in her place, there aren't any big revelations, just small chit chat.
In fact even most small details about the girl you discover are covered in main story plot anyway.

1) First change is a little heart to heart chat after first selection of the 5 girls in chapter 7 after PTSD scene.
Meiya lacks unique scene BUT regardless of whom you choose there will be a heart to heart with Meiya one on top of whichever other one you chose. Choosing Meiya on that one does essentially nothing.
Tthose are easy to see for every girl by simply saving on the girl choice menu if you only want to do 1 ending.

2) Second changes will be in 'Extra' world revisit - you again have to pick the girl.
During the period the changes are:
A bit heart to heart with the girl of your choice and small dialogue choices along the way due to it, all play out almost 1:1 identical no matter the girl.
Slight change to final yuuko scene depending on you choosing meiya or not (If meiya does not forget you, meaning not selected her - you ask yuuko why not borrow nuclear station from Meiya.. otherwiseit will be Tsukuyomi.

3) A small heart to heart on the ship towards Sadogashima.
This one plays out near 1:1 identical yet again, if you have seen one - most dialogue from takeru side will repeat and the girls will generally be speaking almost same stuff in different manner.

4) EXTREMELY slight changes to how Meiya scenes play out after the Isumi Funeral depending on girl you selected being Meiya or not. Honestly almost 1:1 identical.

5) Short remark to herself from the girl you chose about Takeru having 'no tact' during scene where Sumika visits him before final battle and them getting all close without disabling transmitter.
Fully 1:1 identical for each girl, only girl who speaks changes.

6) Arguably the best/most impactful scene change - a 'farewell/will' letter from each girl during final meeting with Yuuko after the finale.
Needless to say they all write about their love they couldn't confess in person.

If you picked Meiya there isn't one since she assumingly didn't write the will(conveniently her own finale is a main story that kinda reveals all her feelings regardless of whom you pick), instead you will have a bit more dialogue with Kasumi if you picked Meiya.

Due to how long it takes to reach those even with autoskip due to all forced animated scenes and it being essentially the only interesting scene change - I recommend simply checking those out on youtube, will leave links below.

Chizuru - https://www.youtube.com/watch?v=OQwNQcLB-lw

Ayamine - https://www.youtube.com/watch?v=6A_NB6Jg59E

Tama - https://www.youtube.com/watch?v=uNB4kleI6F8

Mikoto - https://www.youtube.com/watch?v=MkGn4S0SCts