Doesn't matter. Only time where I could say it matters is if the person reading your app personally knows the person who you did research with. This is more likely if your PI is a big name person (so most likely a full-fledged professor).
Outside of this, I can't see it mattering much - what is MUCH more important is your PI being a nice, genuine person who is willing to be a mentor for you (not just treating you like a workhorse).
That being said, it may be unfortunate if you work with an assistant professor, and then after a couple years, he/she does not get tenure... you might get left out in the cold and have to find a new PI in that case... just do your research and find someone you're interested in!