Front-opening: More convenient (though this may be a bit of a subjective comment). Rather than having to reach in from the top, you can reach straight in through the front... When you have to move things, it's horizontal movement rather than the more precarious vertical movement. It also helps when dealing with the snakes themselves... Not having to "lift" heavier/skittier animals; being able to "scoop" instead... For that matter, a hook is much easier to maneuver with a front-opening enclosure than it is with a top-opening if you have an aggressive and/or massive snake. (Oh, how I wish I had a front-opening enclosure for my Beauty. I think it would cut the stress of hooking her out of her tank in half!)
Top-opening: Don't have to worry about setting things against/in front of the enclosure. Don't have to worry about doors getting in the way (though where you put the tank lid while fussing with the tank is a different matter). Can rest/attach branches/decor to the front of the enclosure, rather than carefully tailoring decor so it doesn't have to rely on the front wall. (This really is the only downfall of my larger front-opening enclosure... I would much like to turn the large branch around for the top to be at the front, but, by the very nature of a front-opening viv and the style/weight of said branch, I cannot.)
Given the choice, I'd go front-opening all the way. I only have two at the moment, but they're definitely a million times easier to deal with than tank enclosures. (IMO, at least.)