Hey coooper,
DISCLAIMER: this post is a reflection of my OPINIONS. I am not suggesting any of the below to be factual, and much of it is based on hearsay. The writing about how great Newman is is all true, but I can't say for sure whether the stuff about the other colleges is. Thank you.
I think you'll find that just about all the colleges will have a great atmosphere, and that you'll enjoy your time at any of them (apart from St. Hilda's, which I've heard bad things about). I wouldn't doubt that you'll actually develop a pretty strong bias towards the college of your choosing. I'm at Newman, and certainly have. As far as I'm concerned, it is by far the best of the colleges. The grounds, buildings, staff and fellow students are wonderful, and I am having the time of my life. Being one of the Big Four (Newman, Queen's, Ormond and Trinity) adds a great deal of tradition and prestige as well. If you're looking for an awesome place to spend your uni days, Newman won't let you down.
That said, I think that any of the Big Four will offer similar experiences, quality-wise. As they attract the most applicants, and therefore enjoy a higher calibre of resident, their O-Week programs and range of activities available will probably give you the opportunity to really bond with others in your cohort, and get stuck into areas you wouldn't otherwise have had anything to do with (I joined debating recently, for fun). If you feel confident that you have a vibrant personality that you can showcase at your college interview, and a specific skill that you can offer as your sales pitch (however, remember not to brag at your interview. Just answer the questions honestly, know some of the history of the college you've applied for, and talk about your passion for both the college and whatever your interests are), I'd definitely go for one of the Big Four (I've heard that Trinity is quite sporty, so take that into account). If you're not confident of these, however, it might be better to try for one of the smaller colleges, most of which seem fine.
St. Mary's tends to attract good-looking young women, so that might be nice if you're a guy. Be warned, though, that Mary's always plays second-fiddle to Newman (we are neighbours, and so share the Academic Centre, among other things). The building looks quite drab from the outside, but then again, so does Newman, so perhaps it's OK inside.
University College, being free of religious affiliation, is known for the ridiculous amount of sex that occurs within its walls. Apparently it gets pretty wild. I was thinking of applying here (long before I knew about their sex legacy!) before I decided to choose Newman instead, because I had a friend who attended and said he had an awesome time.
International House is seen as the bottom of the pecking order, and incredibly easy to get into. It's a great one to list as your second or third preference, because as long as you're not dysfunctional in some way, you'll probably get in. Also known for its multitude of Asian students.
St. Hilda's is apparently not much fun, according to residents. Which is a shame.
Janet Clarke Hall I don't know much about, but the freshers I spoke to from there said they had an awesome O-Week and that the building is very nice. Note that it's further away from the uni campus than most of the other colleges, which might mean you wake up 5 minutes earlier.
Whitley College is another one I don't know much about, sorry.
Medley House has been closed since 2009, but is re-opening in semester 2 of this year. I'm rather excited to see what next year's intake of freshers are like, because no one seems to know anything about Medley.
Rules-wise, Newman is considered to be one of the strictest colleges, but you'll find that, as long as you're discreet, many of the "rules" can be bent, and usually you won't get in serious trouble for infractions. They're meant only as a way to punish people if admin so desires. But you'll find out all about that in your O-Week, if you come. I guess I'm saying that you shouldn't let the strict rules dissuade you from applying to Newman.
Like I mentioned, I imagine you will have an extremely good time at, and become very patriotic towards, the college you choose, so I wouldn't worry too much about your application. If you're set on getting in (you don't want to rent-share with anyone), I would probably recommend you apply to one of the easier colleges (Newman receives about 500 applications every year, of which about 150 are successful). Although, many of the colleges set aside a group of spots for second-preference-applicants, which may increase your chances. I know Newman does this, so if you want a fallback from one of the other Big Four colleges, you could put us as second. Occasionally, colleges foolishly reject high-quality applicants, which is why we have spaces set aside for people who get knocked back from their first preferences. Several of my friends got in through this method.
Also worth considering is what university you will be studying at. If it's The University of Melbourne, then there's no issue. But, if you're studying at a different institution (RMIT, ACU, Monash Pharmacy School, etc.), you'll need to choose your college preferences with this in mind, as not all of the colleges accept applicants from outside UniMelb/VCA. A breakdown of which ones do and don't is available here:
http://www.colleges.unimelb.edu.au/information/nonuomadmission.html
Also important is that the religious affiliation of the college you choose should have very little bearing on your choice. Religion is not forced upon students in any of the colleges and, for example, only about 50% of residents at Newman would identify themselves as Catholics. The colleges accept people from all faiths (or no faith), and the religious ceremonies will only ever play a minor role in your college life, unless you want them to play a larger role. Each of the Big Four will probably have a compulsory church service every now and then, and say grace before formal meals, but that's about it.
Good luck with your choices and your application, coooper! If you'd like any advice on interview questions (specifically for Newman, although I imagine the types of questions asked would be similar regardless of the college), feel free to send me a PM.
Cheers,
Franklint