If you don't do any editing; The difference in quality is present but not much
What you need to consider though is
You will need a decent Specified PC to view them
You will need ( perhaps) to get An external HDD to "store" the Video footage unless you want to rely on Your PCs storage
You can Play them off the camcorder to TV but when it is full, youll have to remove what is on its HDD in order to free it up to transfer older footage for viewing
While it may be possible to get domestic recordable Blu ray/ HD DVD discs in the future, for now turning the footage to DVDs you can play will involve at least capturing, turning them to Mpeg2 and then authoring a DVD. You will find it impossible to avoid PC software
For your family ( once a year videos) I imagine you may not want to bother with authoring; For the Gymnastic competitions....you will
To compare quality
this post by Redsox mark has link with a mix of footage from miniDV and AVCHD captured and converted to mpeg2