Re: i bought the lactic acid…
Here are the reasons I personally switch:
Lactic gives my face a brightening effect that I don’t get from glycolic (again, this is just how MY skin reacts). Both the peels help even out my skin tone and make it look smoother…but lactic gives me a brightening effect.
In addition, over time I’ve learned that with lactic peel…my skin can handle Retin A even the day before and after the peel (ie. I don’t have to stop using it b/c of the peel). But with glycolic, at least with my skin…I can’t use RA at least for 2 days before and 2 days after the peel (skin gets irritated if I do). Same with my vitamin C serum…I can’t use it the day before and the day after glycolic peels.
This is why I alternate so my RA/Vit. C serum usage isn’t interrupted too often. I see my glycolic peels as actually doing the job of helping with fine lines, hyperpigmentation etc…and the lactic peel as “maintenance” in between the glycolic peels. And since the cost is cheap…it doesn’t bother me to just have both. 
With stronger peels like TCA…its probably best to have “maintenance” peels like lactic or glycoilc in between. But with glycolic and lactic…there really isn’t a “reason” behind alternating except personal preferance.