We would prefer to make our new site in a seprate site collection for the following are the reasons:
1. Site quotas is one of the reasons. The issue is the recycle bin is based on site collections and the quota for a site collection. If everyone shares a site collection, then they share the recycle bins storage size.
2. Delegated Security and distributed administration is the next big thing. For eg, you have a IT department that doesn't know who should be able to see what content, besides how it should be organized. This is the job of the content owners and users. SharePoint site collections offers IT the ability to create a site collection for a project, team, department, document, or whatever the needs are, then assign an owner and hand it off to them.
3. In addition to these two if you need to separate the content between databases because of space issue.