As the title states, this manga will inevitably be compared with the other series, and alas, this one is just... worse. That's not to say it's terrible; if you like the author's other work Aho Girl you'll probably like this. But again, it's just... a lesser copy. I'm trying to be gentle about it, because it really isn't terrible (provided you like this type of manga at all, but if you don't, then why are you even here?).
Honestly, the main difference between the two manga is that here, everyone is just stupid in general, whereas in the other series they're the epitome of 'baka-couples' - it's also gag based, but all the characters are just wearing incredibly rose-tinted glasses. So they're stupid for eachother in a way that makes you laugh, but here everyone's just... dumb. Frankly after reading the two side by side it was a mistake, because it only emphasizes the flaws of this one lol.
Let's compare MCs. In 100GF, the MC is honest, forthright, and devoted to his GFs. He's not actually dumb, though, but the old adage 'love is blind' or more perhaps 'love makes you do stupid things' is very apt lol. Here, the MC is honest, forthright, STUPID, and honestly pretty degenerate and weak willed. But he's very self aware about it, and tries to mitigate it. So again, were it not for the fact that the other MC is just a step better in every category, he'd actually be pretty tolerable!
A key difference between the two harem-collection methods though is that in 100GF, the MC is straight told by the god of love that the reason he couldn't find love was because the god himself screwed up, and put all his life's worth of luck into romance, and he now has 100 soulmates. Key note- his soulmates will actually DIE if he rejects them. Make no mistake though, there is much love in every direction, between every particpant, but the initial impetus for forming a harem was to save their lives from something out of his control.
Here, this MC does it because he's weak willed and greedy, even if he's honest about it.
Some other comparisons between the two: there will be 100 girls eventually in 100GF, of which there are 10 (?) by chapter 45. Here no clue how many, but it won't be that many.
Both are gag based manga, but 100GF loves to include references, copyright jokes, and break the 4th wall often. Oh and the author seriously trolls the readers sometimes with complete crack chapters. I don't find this one as funny, though it is still pretty good.
Your mileage may vary, but by their very natures the harems will have someone for everyone. I'm sure you'll find some you love in 100GF strictly by sample size lol. Here the shipping wars will probably drive you insane.
Some cons that you may find with both:
100GF I'm wondering a) if it'll ever finish, and b) how it'll turn out when more girls are around. They're in school already with a group of 11ish people, and easily manage to have them all participate in the story. When there's 50, much less 80, much less 100? Not sure how that's going to go.
For Kanojo mo Kanojo... the shouting is incredibly annoying. Quite literally 90% of the chapters have everyone shouting. I can't un-see it, and the fact that every character is so dumb really makes the enjoyment lessen, much like Aho Girl did. Mangaka to Assistant didn't have that problem for me, so I hope it's not that the author is just writing this way from now on...
TL;DR Both manga are ridiculous harems built on ridiculous premises. Both gags, both with earnest MC and girls that are willing (or semi-willing in KmK's case) to be in a harem. It's a shame they're both so similar, because it only emphasizes the flaws of the lesser one (this one). My suggestion is NOT to read them near each other, and if you only want to pick one, pick up '100 GF who really really love you', not this one, alas.
But again, it's not bad. Just worse than the competition lol.