It's always a fine balance deciding between quantity vs. quality... too much of either makes it too easy to abuse.
With current categories of:
W - (counting, favoring quantity, encourages more starters)
L - (negative, favoring quality, encourages less starters)
ERA - (average, favoring quality, encourages less starters/more closers)
WHIP - (average, favoring quality, encourages less starters/more closers)
K - (counting, favoring quantity, encourages more starters)
SV - (counting, favoring quantity, encourages more closers)
you're putting together a league that could already be slanted slightly towards small staff of elite closers (with an Ace thrown in for good measure, if possible).
K/9 is yet another averaging category that will favor elite closers and aces only.
Holds is an interesting choice, because it opens up a lot more options on peoples' rosters and adds more strategy into the planning of your team.
But, my favorite here would be QS. Yes, it is somewhat redundant, with W and L already as categories, but it's really the only thing that encourages players to start anybody except aces and closers. This way your 3rd- and 4th-tier starters do more for you than just pile up Ks and drag down your L, ERA, and WHIP.
If you're looking to add more players into the player pool (in a large league this is definitely a good thing), then I'd go with Holds.
In a small/medium league, where you're looking to encourage more people to play the pitchers in the existing pool, I'd go with QS.
Leaving it the way it is, or even moreso if you added K/9, people will be strongly tempted to draft one ace pitcher and four closers and only start them the entire season, not willing to sacrifice the huge edge that gives them in 4/6 categories (or 5/7) just to make up for the other two (W/K).