Greetings, and welcome.
Spyder said it well so +1 to most of his comments, particularly with regard to the Titleist AP1s. I replaced my Ping G20s (yes, I also understand your comment about the G15s feeling clunky) with 714 AP1s a couple of months ago and couldn't be happier.
You said you were open to new or used clubs, but I've no idea of your financial position of course. The thing with fittings is that a good one includes trying various shaft combinations as well as determining whether you need R-flex or S-flex shafts and measuring for lie angle. I was amazed at how much variation in feel and ball flight you get with different shafts. So if you can see your way clear from a money perspective I think a properly fitted new set would likely be the way to go.
If you are thinking of used, I'd at least go for a big-box store fitting of the brand and model you're looking at. They may not have anything but the standard shafts to try, but at least you can determine flex and see how they feel before heading off to shop Ebay or whatever.
Finally, I personally think that mid handicappers are much better off sticking to game improvement clubs rather than diving into "players" models, even if they are pretty and feel good on the occasions you hit one cleanly. There are a range of GI clubs, the Ping G15/G20/Karstan being out on the end toward Super GI, and the AP1s some Callaway Apex, and others being closer to the "Players" side of the spectrum. But golf is hard enough without saddling yourself with clubs that are harder to hit than need be.
How to decide? One valid way, IMHO, is to talk to your local pro and other good golfers you may know and ask who the good fitters are in your area. The biggest reason my bag is stuffed with Titleist clubs is because there is a Titleist Advanced Fitting Center with an excellent fitter in the area. Find a fitter you feel good about and then select brand/model based on that person's recommendation.