Boman has pointed out that the earliest extant Suetonius manuscript contains the spelling Christiani, rather than Chrestiani. Since the oldest extant manuscript of Tacitus refers to Chrestiani, and since the Codex Sinaiticus originally contained references to Chrestiani, would it not be possible, therefore, that Suetonius originally wrote Chrestiani but that the copyist of the earliest extant manuscript or a previous one made the "correction" to Christiani?
Of course, if Tacitus is referring to Chrestians, then his "Christus" would originally have been Chrestus as well, if the sentence following the discussion of Chrestians is even genuine. In reality, it appears to have been interpolated into the text at some point between Tacitus's writing of the passage and its appearance in the earliest manuscript of the Annals, whence come of all others. The introduction of "Christus" into the paragraph makes little sense if it is original to Tacitus, since the Chrestians would not have been named for him. It is possible that the following clause, therefore, is an interpolation:
auctor nominis eius Christus Tibero imperitante per procuratorem Pontium Pilatum supplicio adfectus erat
This contention would effectively remove Tacitus as a "proof text" for the existence of a "historical" Jesus of Nazareth. The fact is that the awkward juxtaposition of Chrestians and Christus makes no sense and that it may not be inaccurate to assert that our earliest manuscripts all possess interpolations or other alternations by Christian scribes.
Ditto with Suetonius, who refers to "Chresto" - it would follow that he mentions also Chrestians, although he does not connect the two passages. Outside of the NT and other Christian stories, we simply have no solid evidence of Christians at Rome during Nero's rule, no contemporaneous reports, no inscriptions, no tomb artifacts - nothing. It seems Suetonius would know that fact and not write "Christians."