I've been disappointed with the post-Infinite Crisis Wonder Woman. When they first "relaunched" her she was supposed to become in most respects an updated version of the Lynda Carter Wonder Woman-as-secret agent version of the character from the TV series. There was this whole bit about her joining a top-secret organization (not too far removed from Checkmate), and there was a tantalizing plot thread, never to my knowledge followed up on, that her job might require her to kill people - this in the post-Maxwell Lord era, and also at a time when Checkmate was exploring the morality of "ends justify the means" though DC wussed out and dropped this from the Checkmate title before doing much with it.
But within a few issues, Wonder Woman went sideways. There was an overlong story arc involving some sort of netherworld that was just boring (it didn't help that my local comic shop for some reason actually skipped an issue for no particular reason, so following the narrative was even harder). The whole "secret organization" subplot - including a romance - sort of faded into the background. And then there were the talking apes who, even coming from the company that gave us Gorilla Grodd, were just a little bit shark-jumpish.
I ultimately gave up on WW. These days I might buy an issue if the art is particularly nice, but otherwise it lost me.
Power Girl, on the other hand, has managed to keep focused. Anyone familiar with the John Byrne era of She-Hulk will see PG is a kindred spirit to Jennifer Walters (in fact I wish we were back in the days of Marvel-DC crossovers because I'd love to see a Power Girl-She Hulk crossover). The only thing they haven't done is break the fourth wall with PG (one of She-Hulk's unacknowledged powers, during Byrne's era anyway, is the fact she's the only Marvel superhero who is actually aware that she's a character in a comic book). There's been a lot done with PG's character since Infinite Crisis, and so far the comic hasn't gone sideways. My favorite story arc so far (and this isn't a spoiler, really) is the one guest-starring Sean Connery from Zardoz. Believe me - if you know the movie you'll recognize it when you see it!
So I definitely recommend Power Girl over Wonder Woman these days. It's simply more entertaining and they seem to be sticking to plan, as opposed to WW which has undergone almost as many "do-overs" in the last few years as the Legion of Super-Heroes.
Alex