I hereby speak without much knowledge but with some experience. They're both alright. And they could both be much better.
I have played both but GG seems to have something that gets me back to it. I don't know what exactly. Maybe the graphics. Don't know, It's just nicely made. I find PS to be rough and outdated. In microstakes, which is all I can afford to play, players are pretty much the same. GG has the asian gamblers and some regs and PS has just as many gamblers and some regs with HUDs. GG doesn't allow external HUDs, which I also like cause the grinders have less information. GG has bad beat jackpots that are pretty good and the GG Care and the daily micro tickets. Don't know (don't judge) if Pokerstars has any of that. Deposits and withdrawals are acceptable on both, had my winnings in a couple of days. PS software has glitches on my PC, frequent errors and some lag when moving the windows.
I'm the kind of person who enjoyed PKR and It's just little things that make GG more playable for me. I think PS is pretty down on my preference list and I simply don't like it much. If there were only worse alternatives (and there are), I'd stick with PS but there are better alternatives for me. I prefer
888, GG, Party, even ACR (actually on the edge with this one). But I prefer PS over everything else. I may also not have enough experience with other poker clients/platforms to compare them, but I've tried about 20 different ones. Some attract me, some don't, very subjective.
Microstakes players are just microstakes players, the more the merrier, and GG's and PS's pools are both big.
Depends what concerns you and what you personally prefer. Not much to be concerned about with either. There are many people who will argue about anything, but it comes down to where you feel comfortable and what attracts you and the only way to find out is to try out everything that's available and then choose for yourself. Or don't choose and play them all at the same time or each as your mood pleases. But play well.
Check the rake too, if it concerns you, it's readily available. Rake can be annoying, but I see it as a tax and I pay my taxes as there are no tax avoidance schemes in poker.
However, I think it's more productive to focus on winning rather than rake as if you lose consistently, the rake is literally your last concern.
Keep it simple.