From my experience, it appears that modern works on old philosophical ideas are easier to understand, have updated/modern language and examples and add relevant objections/support from modern science. The original works seem to suffer from redundancy and, in the case of the ancients, awkward ways of going about a philosophical investigation.
In saying that, there are philosophers that no one appears to agree upon what they actually meant. Everyone still argues about Nietzsche and there are dozens, if not hundreds of ways of interpreting Marx. Reading abridged copies and modern textbooks on these guys seems like an easy way to set yourself wrong.
what are you hoping to achieve in your reading?