Question: * What's the limitation?
* Is there a specific volume of data each can handle regardless of disk space?
* When to use what assuming licensing is not a problem?

Answer: 
I'd say if you want to compare apples to apples, then it is MySQL vs SQL Express, vs Oracle Express.

Or if you have $, then it is the MySQL support license, MS-SQL Standard, vs whatever Oracle's cheapest offering is.

In my experience, once you choose a language, e.g. Php goes best with MySQL, then you've chosen your DB. Java goes well with Oracle. C# goes well with MSSQL.

Similarly, if you choose your OS, then unix flavors run MySQL or Oracle, but MSSQL is windows only. MySQL and Oracle work on both unix and windows of course.

[If you need to buy many machines, then not having to pay OS licenses for the server helps in scaling](https://blog.codinghorror.com/scaling-up-vs-scaling-out-hidden-costs/).
Is the application buy or build ?

* If Buy, does it support all three and
talk to the app vendor about the
differences ?
* If Build, then is it an in-house
build or contract out. If contracting
out, put out your requirements and
let the suppliers put their
arguments.
* If in house build, then first look at
why you are not contracting out.
Normally it is because you already
have an in house capability, so look
at that expertise.

You want some sizing information first.
Are you talking data volumes in megabytes, gigabytes or terabytes ?
What are your uptime requirements, backup (recovery time / recovery point) ?
How much concurrent activity ? Is that peak ?

Generally any database system is fine for data storage and retrieval. High-end analysis, load balancing, replication, management, backup/recovery, auditability, security are all areas you may need to consider.