A transponder antenna will be about 1/4" shorter (2.659" vs 2.408") than a 978MHz antenna, so shorter but not by much.
Even though others here will swear it is a disaster waiting to happen, for our application you can use it with no problem. Now, if you are in a controlled laboratory environment with sensitive measuring equipment then you will notice an improvement with an antenna cut specifically to 978Mhz instead of 1060Mhz. But with a good installation you really won't see a difference in operations.
FWIW I have been using a regular transponder antenna for my 978UAT receiver since 2013 and get the same reception as others, flying with specifically designed ADS-B antennas. At the end it comes down to a decision based on costs, ease of installation and looks more than anything else.