Her professor was convinced that you cannot take a picture, unless you have the most expensive camera, the most expensive lens and the most expensive equipment.
This is a common misconception, even if not entirely false. I would rephrase and split as three distinct concepts:
- At some point, you will feel limited by the technology
- A more expensive equipment is not necessarily easier to use
- Within a reasonable price range, more expensive does not always imply "better"
Ken Rockwell is a huge fan of the Tokina 11-16 F/2.8 (yes, it is sold also with a Sony mount)
This lens is 20-30% cheaper than the (indeed excellent) Sigma 10-20 wide angle, and has similar or better performance (just google for "tokina 11-16 vs sigma 10-20")
Sometimes it may be worth to invest in a versatile lens that does everything uniformly well, but nothing excellent, for example the Tamron 18-270