I can tell you a few limitations of Standard and some things to consider to see if migrating to professional is the way to go.

1. If you need to have multiple workflows, professional allows this. This is if different groups of employees in your company need to have different approval routings for some reason.

2. If you have or want multiple ledgers for your expenses.

3. If you want to use alternate account codes for expense types and have them tracked for more than 10 of your company entities. For example, if your company uses Department to track expenses and each department has their own GL Code for expenses. Standard edition allows you to have alternate codes, but for only 10 departments (or whatever custom field you need). Professional allows for much more.

4. If you need to have different policies for your employees. Policies determine how a user will "use" the system. Standard doesn't really have Policies like this.

These are just a few things to consider. There are others things to take into account, but these are some of the most common.

I hope this helps.

Kevin