main diff is we can upload the docs in to qc.

The main advatage with QC is we can upload all the docs 
including defect report and QC can be accessible by all the 
employees the company,
If you use Excel sheet you need to mail that particular 
sheet to concerned person as well as related persons who 
involved in that project if necessary.

The main advantages are

1. we can map the requirements to the test cases  to check
if we have covered generating test cases for each and every
requirement which we cannot map in excel sheet.

2. Can upload test cases and docs very easily.

3. When bug found during execution we can easily know which
line it has failed with full details we can even take
snapshot of the bug easily which cannot be happened in excel
sheet.

4. N no. of users can use this

5. We can check the % of test cases executed and which are
not which is not possible using excel sheet that doesn;t
display any % of runned test cases.

6. We can see chart representation for the executed test
cases or bugs which by using excel sheet cannot prepare char
representation.


Hope this might give you some idea.