should I choose the university that fits me better or the university which is more selective
Assuming you're lucky enough to have that choice....
You should choose the environment that gives you the greater likelihood of future success, as defined by your own academic and career goals.
This may be correlated with the overall reputation of the university, but any such correlation will be very loose. The overall reputation of the department is a better proxy, but still loose. The reputations of your (probable) advisor or advisors, or even better, of their former students, is even closer, but still not the whole story, because not every student wanted to do what you want to do.
Strong universities have weak departments and vice versa. Strong departments have weak research areas, or toxic work environments, and vice versa. Strong advisors in strong departments may have no interests that overlap yours, or may have working styles that badly clash with yours, or may have insufficient funding to support you, or may be located in places that you find unlivable for financial or cultural reasons.
does the university evaluation (acceptance) mean that I am capable of performing perfectly in their program !?
Of course not! There's no such thing as "performing perfectly".
But acceptance with funding usually means that the faculty—or at least the admissions committee—believes you have a strong potential for success in that graduate program.
(I write "usually" because a few lazy/unethical departments do intentionally accept more students than they reasonably believe can succeed. Especially if this is a serious concern, talk to current students in the department before you accept. And acceptance without funding only means that they're willing to take your money; run away fast.)
Finally: Do not listen to the Impostor Syndrome.