I would say it's depend on what type of MC it is but with my type of personality, I'll go most likely with guy B why? it's easy because of the keyword 'faithful'
Growing up, I learned the difference between being 'faithful' and 'loyal' being loyal means that when another person comes along, they may look their way, they might think to pursue the other person, but they will still RETURN to you. Faithful on the other hand is a different level, when another person comes along, they won't mind the other person who just came along because they themselves decided to CHOOSE you in the first place and they will still choose you no matter what, and by choice, I mean not in a toxic way, but rather he knows what's best for both of you to compromise.
I'm just telling these points based on my past experiences but I hope it may help:
If your MC is a type of woman who is usually described as 'independent' like me, probably, most guys may admire you but many won't approach you just like that because they will most likely feel 'intimidated by you.(yes, it still happens to me until now)
In my experience, the type of guy who will most likely, always make the first move is Guy A.
If the MC was single for a long time and doesn't have much experience in dating, at first she may feel 'special' when guy A approaches her because the first impression will most likely either be: "How could this guy who has everything, noticed someone like me? and decided to approach me, I don't know what to do" (I know I'm guilty feeling that before lol) or "This guy seems suspicious and bold for approaching me, I need to keep my walls up" -- and on either response, Guy A would more likely be persistent and since he is an 'ideal man' he would most likely deal with your response 'the ideal way' Sure, you're going to be treated like a 'princess' (or whatever you want to be treated as) because he is an 'ideal man who has everything' but soon you will realize, his character is pretty bland,and in the long run, that guy A only thinks of you as a possession or a 'flower on top of a mountain that he has to obtain' to the point that he will bet on everything 'he had' for you because he knows that you wouldn't want someone to give up everything for you. Guy A most likely treat you as a possession and it may seem that he treats you as a treasure but someone who is possessive is most likely NEVER genuinely in love at all, Guy A is just in love with the IDEA OF 'LOVE' -- while having a possessive guy may sound 'cute' or can make others swoon for them IN FICTION, but in reality, I will tell you one thing, if you experience it first hand, It's actually NOT ROMANTIC AT ALL sure it's fun to see your 'loved' one jealous from time to time, but jealous to the point of being possessive not at all... it's something I learned the hard way when I was young, and yes I value the time being single after that because I was finally free.
Guy B on the other hand, would most likely be that one person you have known for so long, who has been in love with you for a long time but they are afraid to let you know -- which is why it was always late when you catch up on their feelings.
( just like that guy friend I know for so long who took 9 freaking years just to confess to me, and I have no idea when the hell did he started liking me, also that guy is indeed flirtatious, he did have many girlfriends before finally confessing so how the hell would I know if he's being serious?)
if that happens, MC should probably make the first move (because it's already 2021 lol) or since he is flirtatious, to begin with, he probably has that one gesture that he does only to the MC so the MC should be observant. BUT It would also be interesting too if MC will respond by flirting back unexpectedly (because usually, MC just brush it off) when guy b is flirting with her to let guy b's guard down for a moment and be flustered (I tried that, it was fun to see that guy get flustered lol)
To sum it up, think of it like this, Guy A is actually a possessive guy hidden by the word 'loyal, ideal man, and 'genuinely' in love Guy B is actually a faithful guy hidden by the word 'flirtatious, annoying, womanizer'
besides, wouldn't it be fun to add that twist?