When performing high-level estimating for building a release plan for an agile project, the common practice is to use either Ideal Days or Story Points as your unit of measurement. You may use other techniques (such as Function Points) but these two are the most common.

While both methods will work well for your project, there are differences in the benefits of the two approaches. Currently, the agile community is trending strongly towards Story Points as the basis for initial estimates. There are several advantages over Ideal Days. Here are the major ones:

• Story Points are faster to estimate
• They are not easily confused with schedule commitments
• Story Points self-correct (i.e. are recalibrated) as you move up the learning curve and achieve higher velocities later on in a project
• They can be "bumped" up to adjust for risk, requirements stability, resource constraints, and other factors.

So, don't think that it doesn't matter which estimating basis you choose—there are some significant differences between them. Current practice, however, seems to favor using Story Points.