As you note from the Wikpedia articles, the scholarly consensus is that Caesar did not write these works.
The Loeb Classic Library edition to these works and one other (Alexandrian War. African War. Spanish War) gives some brief examples of the reasons for this, noting that doubts can be traced back to the 2nd century A.D.
Hirtius would seem a likely next candidate, since the style of another "campaign" work (De Bello Alexandrino) is remarkably similar to the book that he appended to De Bello Gallico. Hirtius writes the following at the beginning of Book VIII of De Bello Gallico:
Caesaris nostri commentarios rerum gestarum Galliae, non comparantibus superioribus atque insequentibus eius scriptis, contexui novissimumque imperfectum ab rebus gestis Alexandriae confeci usque ad exitum non quidem civilis dissensionis, cuius finem nullum videmus, sed vitae Caesaris.
I have tacked a supplement to the Commentaries of our great Caesar on the operations in Gaul . . . and his last work (i.e. the Civil Wars), which was left unfinished from the operations at Alexandria onwards, I have completed as far as the conclusion, not indeed of civil discord, of which we see no end, but of Caesar's life.
The introduction to the Loeb edition, mentioned above, does not think this is plausible:
This certainly appears to confirm what seems likely on stylistic grounds, namely that, if it was Hirtius who completed the Gallic Wars, it was Hirtius also who wrote de Bello Alexandrino.... Was he then also the author of de Bello Africo and de Bello Hispaniensi? His words—'as far as the conclusion of Caesar’s life' may indeed be so interpreted. The internal evidence, however, seems strongly against this theory, and suggests, on the contrary, that the three works are the independent productions of three separate hands, none of which was Caesar’s own.
The introduction then goes on to mention how certain "idiosyncrasies of style" display themselves in one book without being in the other. Here are the provided examples:
The author of de Bello Hispaniensi:
- Frequent quotations from Ennius
- Frequent use of bene as an intensive particle, e.g. bene magna pars
The author of de Bello Africo:
- Frequent use of the word interim, such that he "can seldom think of any alternative with which to introduce a new chapter"
Because these literary traits are unique to each, and not present in any of the other works, scholars conclude that they were not written by the same author, at least as they have come down to us now.
They propose Bouvet's suggestion in his edition of the Commentarii, which suggests that these works are "no more than rough drafts prepared at the request of Hirtius by two separate soldiers who fought in the respective campaigns; and had he survived, Hirtius would have worked up this ‘copy’ into more effective literary form."
This last possibility is reiterated later on in the Loeb introduction:
The careful chronology and the faithful record of the feelings of the troops suggests a soldier - possibly a junior officer - who was on the spot. That he was young and inexperienced; an ardent, but not always a balanced, partisan; a keen observer of all that went on around him, but without access to the inner counsels of his Commander-in-Chief.