I went through the same decision process this fall. I had been commuting for years carrying everything on my back, either via a messenger bag, or a laptop backpack most recently. But, as I get older I was getting more and more tired of the weight on my (sweating) back, and being restricted in how much i could carry. I went back to panniers (after about 10 years of back-packs) and am really happy. I have one pannier for my computer stuff (including a heavy laptop like you), and another pannier for clothing and lunch. What is really nice, though, is that I have enough room to bring several layers of cycling clothing. Here in Idaho, the fall mornings can start off cool in the 30's, but be up into the 60's / 70's by the time I'm heading home. No problem, just stash my cool-weather gear into the pannier for the ride home and pull out my short-sleeve jersey and finger-less gloves and I'm good to go. I also don't need to worry about how large my lunch is or bringing in a fresh towel for showering. It works for me!

Also, another motivator to switching to panniers was giving me more flexibility in being able to add some distance to my commutes. I can turn my 5 mile commute into a 15-20 mile training ride without worrying about the extra weight on my back. If anything, I'll pack a Camelback with me in the morning and use it in the afternoon for a longer ride home. Again, I didn't have that flexibility when I was trying to carry everything in a backpack.