I've never been a student at Purdue, West Lafayette, but I have been a student at both IU, Bloomington and IUPUI at various points are they are completely different experiences.
Yes, IUPUI does give a Purdue degree in engineering, but both the diploma and transcript will say Purdue - Indianapolis. When writing resumes, students in Bloomington and West Lafayette are told to refer to their schools as IU, Bloomington or Purdue, West Lafayette to distinguish them from the regional campuses.
IUPUI is primarily a 'commuter campus', I think that is changing somewhat, but is still true enough that it will be an entirely different experience from being on the West Lafayette campus. At IUPUI the average age of students is going to be much higher. You will find a large percentage of adults who are returning to school part time. In West Lafayette, you would find students over 30 few and far between, but in Indianapolis, especially in evening classes, you will find classes that are 50+% comprised of 'older' students. Also a larger percentage of the younger students do commute from their homes and aren't as involved in campus life as students on the more residential campuses.
I am sure some professors do teach at both Purdue, West Lafayette and IUPUI, especially in this day of more online courses where they can do so remotely. But you will also find far more adjunct professors at IUPUI. These are generally professionals who are teaching classes part time. This does have an upside. These instructors have practical knowledge of what they are teaching and can also be very valuable contacts when looking for employment. However once again it is a different experience and having been enrolled in both undergraduate and graduate level courses at IUPUI, I'd have to reflect if I actually took one that I'd call 'challenging'. That could be very different in the engineering school, although I was briefly a student in the engineering department there in computer technology, which is (or at least was) light years away from being the computer science department at IU, B. It often seemed to me that at IUPUI they were teaching for part-time students.
If you are looking to live at home and commute to IUPUI or looking to work in Indy while going to school, IUPUI can be a good option. But be aware that it is not the same experience as being on the West Lafayette campus.
As far as your questions, I have no actual data, but my intuition is that the job placement rates would be very similar, but for entirely different reasons. At IUPUI you should have easy access to getting practical experience while pursuing your degree. Also as mentioned above you should be able to establish relationships with adjunct professors who are famillar with the job market in the area and can help you find opportunities. My best guess would be a large percentage of the job offers for IUPUI students result from internships in the Indianapolis area. Purdue, West Lafayette, on the otherhand, will draw major employers from across the country to their campus to recruit students as it is one of the top engineering schools in the nation.
Once again, not based on any actual data, my guess is that the starting salaries would be slightly higher at the West Lafayette campus. There will be more students relocating for employment and selecting from multiple offers. I think that overall this would lead to a slightly higher average.
It is incredibly easy to switch between campuses though, so if you choose one and decide it's not for you, it would be very, VERY easy to transfer or even to take classes on the other campus to compare the experience.