I’ve heard of Virtonomic before, but I’ve never played it. Judging from the website I’d say the key differences are:
Obviously, the setting is different, with PU playing in a sci-fi world.
The interface is very different, with Virtonomics being very old-school (not a bad thing) and PU trying to make the most of what a browser can do these days.
Virtonomic seems to go more into detail when it comes to the business aspects (huge amounts of low-level stats etc.). While PU focuses on the business aspect as well, it doesn’t try to be primarily a “realistic” game per se, at least not in terms of microeconomics. We do have some things that come close, like income and balance statements and obviously various statistics about how your business is doing, but not on this level of detail.
While not implemented yet, the meta layer in PU will play a huge role eventually. By that I mean the actual competition between corporations for territory and influence. Virtonomics does mention a politics feature on the website, but obviously I don’t know what it looks or plays like.
These are just the most obvious things that I could think of. But without having played the game, it’s obviously hard to tell where the differences lie exactly. But I guess it’s fair to say that they’ll be quite different but similar enough so that someone who enjoys Virtonomics could also enjoy PU