Ok I'd like some suggestion on how to best explain the new penetration. My group and I have been playing Ars for a few years, since 3rd Ed. I have a good group they don't complain much and really put alot of work into thier characters. However with 5th there's an issue one player has with the way penetration works. To me it makes sense and works fine I've worked with him on the reasoning and he still has issues with it and will play with the rules as is.
His basic complaint the way he sees it .."How is it that my higher level spells are weaker and more useless than lower level?"
My explanation is that it's not the spell thats weaker it's the penetration. You put more energy into casting the spell so that leaves less power for penetration. I've also pointed at that this makes arcane connections and talismans needed more.
any thoughts?
Salvete!
I see your challenge there - I myself run a saga begun with the 4. ed rules (although I've owned and collected Ars-material since the 3rd edition. It just took my a decade to finally start to play the game I admired the most). We've gradually adopted more and more of the 5th edition rules - though still not all of them. Some rules we havn't adopted yet due to our preference to earlier systems (combat), and some due to the effort needed to convert (Experience: I've made quite an effort to make a well functioning Excel-based sheet to do all the downtime/adventure book-keeping). Anyway - one of my instant favorites was all of the 5th ed. rules on magic and spells. Not the least the new rules on penetration. Why:
I like the idea that some of the first basic formulaic spell learned by a magus doesn't become obsolete as the magus progress,
I like the idea that often the most simple spell is the way for even an archmage to defeat a tough adversary,
I like that it takes an effort to cast spells and that only increased knowledge beyond what's basically needed to cast a spell is what improves your chance of affecting an otherwise "protected" target (by Might or Parma),
I like that the new more difficult penetration strengthens the importance of the Parma and the importance of its non-proliferation,
I like that the new more difficult penetration strenghten the use of spell mastery - which is much more interesting than in the earlier editions,
I like that the new rules for penetration compel the troupe (and SG) to think of ways to prepare themselves for interesting conflicts (how do we get an arcane connection to the dangerous wolf (with Might) that ravages the covenants countryside? How do we ensure that the battle will be fought on the excact day we've made an horoscope for?) - in other words: the mechanics of the system inspires interesting stories and plothooks!
These are just my honest opinions - don't know if they to any degree can help you persuade your reluctant player. 
And the more powerful spells aren't less powerful than before - they "might" just not be the favorite to defeat Might. 
And if he already knows them - it's just a matter of advancing his Penetration and Spell Mastery skills, and I'm sure that if he know powerfull spells and has a good penetration with them, he would enjoy eternal respect and trembling from his enemies (and sodalis).