I am only new to 68c but i might be able to shed light at your question from a level your at.
I had the same question, i chose to get the designer as i didn't see the point in spending the extra for no reason, and to be honest you can build a great site using the designer edition and even put some mods in, BUT theres always a but.
Some mods require changing your php files, you can't do that except your template folder with the designer edition.
if you have a look at
www.frustratedkids.com , with the exception of the banner which is really ugly at moment i have built with only the designer edition, the banner ads i needed to upgrade to the developer.
I am not unhappy about this because i aim to build many more sites and believe in playing with things to learn and if you have the whole box and dice you can start to make mod's yourself when you know what your doing, but if you do change anything back it up first i learnt the hardway that this is a good idea
i reckon you'll enjoy 68c i have i've only been here a few weeks and learnt a massive amount working with the package and reading the forums
cheers