Custom meta data or custom DB table for data storage?

I’m trying to determine the ideal way to store some user data that will be associated to posts of a custom post type (charities). The data I need to store per user will be collected over time, and will include data such as:

1. Date of contribution (month/year – only one amount per month)
2. Contribution (some dollar amount)
3. Date of update (the date/time this entry was last modified)

This data is collected on an ongoing basis for each user to a single post.

I should be able to not only query which users have contributed to a given charity, I should also be able to determine total contributions to each charity over a given time period.

My instinct would be to set up a separate database table to track contributions whose structure might resemble this:

user_id (int)
post_id (int)
contribution_date (date)
contribution_amount (decimal)
last_modified (date/time)

Or, I suppose I could store this as user meta data, using variable meta data keys like this:

1. custom_$postid_contribution_date
2. custom_$postid_contribution_amount
3. custom_$postid_last_modified

Am I wrong to be considering the meta option? It seems to me that the latter risks getting a bit unmanageable over time and (perhaps?) harder to query for the data I need.