I don't mind either, depending on the writing. Though, I mostly prefer those who falls in love later since I can see the whole process of their relationship
both are the best but i lean a little bit more to the LI already in love. i like seeing him trying to cover up his feelings and being a yearner throughout most of the route. although sometimes it annoys me when hes being pretty obvious but the mc doesnt see it and just chalks it up to friendly behaviour. its also nice because even in other routes they still slightly show their feelings for the mc and may even act on them if the LI of that route isnt treating the mc right. sad to see him get rejected but nice to see the fight between the LI's for the mcs affection
Personally I find it more interesting to watch the process of falling in love, rather than watching someone do things to get the other person to fall in love with them