Hey,
Unimelb student here. Social-wise, I think both unis should be similar - heard plenty of positive experiences from Monash, and have had a pretty positive experience myself at Unimelb too (and Unimelb friends who share similar positive experiences)!
I can understand why you may find the Unimelb environment a bit 'unfriendly' - I guess it's the old buildings that give it a bit of a 'cold' feel. But the people are actually really nice and approachable! And there's a wide variety of clubs too, and many social events like parties which many people enjoy. Personally I am not a social person, and haven't really experienced much of the social side of uni, but my friends seem to really enjoy it and have fun. That saying, I personally enjoy uni way more than high school, because I feel I can be the person I want to be without the pressure of peers/teachers.
Coursewise, I think Unimelb and Monash are also quite similar. Both offer a lot of flexibility, which is great (tho in Monash you don't have to do breadth subjects if you don't want to). Looking at the first year curriculum, IMO Monash looks more interesting content-wise, but Unimelb is still decent. That saying, 2nd and 3rd year at Unimelb are awesome because you pick more specific subjects which have really interesting content!
Since you're interested in doing an Immunology major, from what I've read, both the Unimelb and Monash curriculum look very similar, but Monash does seem to take a more clinical-based approach, which sounds very interesting. I'm sorry I can't give much info, so it would be more preferable if someone majoring in Immuno at either Monash or Unimelb could offer to give some advice!