In the '172' at least...you are flying (sometimes) in the shade.....
And the 'glare' off the 'usually' white wing of the PA.28 can cause you to 'close one eye' when heading north with a setting sun....although when learning, you would be far too 'busy' for that....(?)
They are both stable acft for the purposes of the navex - I have instructed in both - I may prefer the PA.28 because of its slightly more 'solid handling nature'....but when it all boils down...like 'CP' said...Holden or Falcon..??
On a hot day in Turb., the '172' may 'float more' up and down....whilst the PA.28 just 'bumps thru' up & down.....
Final analysis...it doesn't really matter...IMHO..!!
Which aircraft do YOU prefer..??