I'm not Bret Edge, don't have a gallery, nobody knows me, and I probably suck as a photographer compared to Bret, but at the same time I will openly question his suggestion (a bit) that Lower Antelope is best at mid day; I've visited probably a dozen times across a few years, both in May and September, and in my mind, having shot every hour down there from when they open at 8-ish until 2 in the afternoon, I'd argue that the best time for lower antelope is actually two time slots in mid-late May: First and foremost, as early as you can get down there and I'm talking get your ass at the gate so you are car number 1 waiting for the staff to open the gate and then get down there ASAP and don't overshoot the first 100 feet of the canyon like most newbies do. Second slot is to wait until about 09:30-10:00 to start and shoot the 2 hour window from then - that's when you'll get the shaft through the eye (light shafts aren't common in lower). While I've gotten serviceable stuff past noon, I'm of the opinion that the light there, in May, is better before then. If there was a 7:00am possibility I'd be up for that, but there is not at least I guess unless you know someone.
However, I absolutely agree with Bret that lower trumps upper. Never in any of my visits have I ran into a photographer who, after doing both, thought upper held a candle to lower. My other tips are to travel light, it gets crazy tight in the middle of the Lower and you don't need to be hauling around your 800mm prime, ya know?
If you do go to lower, see if Tilman B. is working there that day - he knows the best spots and is an all around great guy to talk to.