Because I continuously buy fragrances .. I buy smaller bottles .. buying bigger is better value for money obviously.. but it's going to take me so long to use them up .. I just get the smaller and if I find that I can't live without the fragrance, then I will buy the bigger bottles. I usually start off with samples .. travel sizes .. 30 then 50ml. U will be surprised at how many your not that crazy about once you start wearing them. If you buy smaller you can afford to buy others.
Definitely get the smaller bottles! Especially if you plan to have a collection, 100 mL would take a long time to get through if you have other options.
Since you are just starting to collect and your budget is limited, I would recommend to go for the smaller bottles. It gives you the change to spread your budget between different fragrances. A small tip: create a fragrance bankaccount where you save each month a certain amount only for fragrances, you have always a clear view of your budget when shopping and I find it easier not to splurge (i speak as a non-student ;-)). have fun collecting and enjoy the ride!