The open source product may be better, but it just won't be used.
Ok, your budget is tight. Open source is free and saves you cash.
Its not the only factor to be considered.
You have to train staff up on the open equivalents.
Various systems will require tweaking to get them to work properly (eg, you could have a bit of software to launch word template documents. It would need tweaking and so would the template files).
Not so many IT people know how to support the open equivalents, so will require extra training or a more expensive outsourcing company.
You know the boundaries of a tried and tested solution from a tried and tested provider. Unknown territory is risky.
Incompatibility issues. Other departments may not be changed over, so documents might not be so easily transferable.