Aside from the fact that the jurisdiction of the these two actions are different; annulment of judgment is with the CA while reconveyance is with the the RTC; the differences were elaborately discussed in the case Martinez v. CA (G.R. No. 170409, January 28, 2008)
As cited in the aforementioned case, the difference between these two (2) actions was discussed in Toledo v. Court of Appeals1:
An action for annulment of judgment is a remedy in equity so exceptional in nature that it may be availed of only when other remedies are wanting, and only if the judgment, final order or final resolution sought to be annulled was rendered by a court lacking jurisdiction or through extrinsic fraud. An action for reconveyance, on the other hand, is a legal and equitable remedy granted to the rightful owner of land which has been wrongfully or erroneously registered in the name of another for the purpose of compelling the latter to transfer or reconvey the land to him. The Court of Appeals has exclusive original jurisdiction over actions for annulment of judgments of Regional Trial Courts whereas actions for reconveyance of real property may be filed before the Regional Trial Courts or the Municipal Trial Courts, depending on the assessed value of the property involved.
1 G.R. No. 167838, August 5, 2015, 765 SCRA 104 [Per J. Jardeleza, Third Division].