Is there a compelling reason to buy the Kehos SA Harav instead of the Oz Vihodor?
Yes. In no particular order;
1 - From what I've seen of the Oz V'hadar, the Kehos one is much clearer and easier to read.
2 - The footnotes. To expand on chbochur's point, they did a wonderful job. First, they made clear which notes came from the Alter Rebbe (or his brother the Maharil), and which were added later. I do not think Oz V'hadar made that distinction. Second, they added marei mekomos for everything. Third, they reference later seforim of the Alter Rebbe (such as the siddur) when he paskens differently. This is often helpful in understand Chabad practice, as we follow the
mishna acharona. Finally they reference later seforim that discuss the Alter Rebbe.
3 - The text was corrected based on early prints and kisvei yad.