SP Upgrade is compatible to transfer data between Joomla versions 1.5->2.5, or 1.5->3.x, or 1.5->4.x
SP Transfer is compatible to transfer data between any combination of Joomla versions 1.6, 1.7, 2.5, 3.x and 4.x.
The reason there are two different extension is because the whole database structure of Joomla! 1.5 is changed and is very different in newer releases. Even though the user interface of the component seem similar, the functionality that lies underneath is not the same. After Joomla! version 3.x appeared, we had to rewrite the whole code from scratch, and is not possible to combine the two.