Ghostbusters Pro seems to be a sturdier, more reliable machine. The catch is that it's an older game utilizing certain components that Stern no longer offers.
Ghostbusters is my favorite film of all-time, thus it's my son's favorite as well. Keeping that in mind, please play both games before you make a decision. With custom audio, Ghostbusters scratches that nostalgia itch in a fierce way. On the other hand there are fewer games that really capture the true atmosphere of a theme like Alien.
Both games are beautiful. Watch the market, especially right now. I'd say jump on a Ghostbusters ONLY IF it's got some extras, if the price is great. Please do not overpay.
Alien is on my wish list and is currently being built and supported.
For the current MSRP, Alien Ripley Edition is probably the way to go here. Ghostbusters wins if you find someone parting with one for 7k or less. Inspect the cabinet, especially the corners; I have seen some people have issues with separation.