I'm torn between the two. I go to Home Depot more often and have purchased more high-ticket items from them (my entry door, flooring for my kitchen, etc). I love their BEHR paint also, and have used it a lot in my home. I find the prices are usually lower at Home Depot, and I have their credit card so they send coupons and special financing offers a lot. On the bad side, their customer service could be better (I once cut my hand trying to buy drywall because I couldn't find anyone around to help me).
Lowe's seems to have better decor items and gifts, as well as better customer service. This summer my friend asked for Garden Gnome's for her birthday and Home Depot literally did not have any in stock. I went to Lowe's and found a ton of them! Their store is brighter lit, cleaner and better laid-out than Home Depot too.