The biggest difference between authors is the gaping chasm between primary and secondary sources; between chaps who were literally (or nearly) there when the ancient events happened and modern writers with access to much more general knowledge but lacking any hope of speaking to eyewitnesses and the like.
Ancient authors are sometimes blatantly partisan. Livy, being a Roman, was rather understandably pro-Rome when writing about the Second Punic War. This doesn’t render his entertaining history worthless, however, as he does an excellent job of conveying the pervading sentiment of the city during the crisis of Cannae’s aftermath.
They did, however, have access to people who witnessed or even participated in events of importance. Thucydides, for example, was a general in the Peloponnesian War and had decades to speak with others who participated or witnessed its moments of interest.
I quite like primary sources, and prefer them to modern ones on the whole. They’re imperfect, often biased and (excepting a few top chaps like Polybius) aren’t necessarily bothered about being entirely accurate, but they’re of the same time as the events they relate, and without them we would not have such a rich knowledge of history.
Naturally, modern authors and their writings are largely dependent upon their forebear historians. However, they do have access to a large number of ancient sources and modern techniques are constantly increasing our understanding of the ancient world (for example, it was found quite recently that gladiators were mostly vegetarian and usually quite chubby).
Some ancient writers are easy to read (Suetonius, for example), but others can be more difficult (such as Thucydides). Modern writers are typically much easier, and aspire to a greater degree of objectivity than some of their ancestors.
Not that there’s a total dichotomy between the two. I’m presently halfway through the third volume of Edward Gibbon’s excellent Decline and Fall of the Roman Empire, which is a couple of centuries old. So, it’s not really ancient, but can scarcely be called modern either.
Anyway, once I finish off Gibbon there’s quite a few history books I’m looking ay getting. Perhaps a biography of Aurelian, or the Gladiator Unofficial Manual. Hmm.