I don't think there is much advantage to one over the other. GM tried to push side post as a better technology, but even GM went back to top post. As mentioned the terminal connection has more surface area on a top post. Side post are easier to quickly disconnect or connect, all you need is a ratcheting 5/16" box wrench.
I think initially when batteries had vented caps the thought was side mount terminals were exposed to less gas and corrosion would be reduced. Shortly after side terminals GM introduced maintenance free batteries. I don't think they were much different than wet cell batteries of the day, but they eliminated the vent caps.
I believe AGM batteries are pretty common on cars today, they vent very little gas, pretty much reducing the issues with terminal corrosion. I went with an Optima AGM on my car for several reasons. Less gassing, corrosion and it has both side and top mount terminals. The manufacturer of my EFI system insists the ECU and CDI box get mounted directly to the battery and not to use buss bars or junctions before the battery. By using a dual terminal battery I was able to retain my original battery cables using the side terminals. The top terminals were open for my ECU and CDI box. If I ever have to go to electric fans I can also connect them to the top posts.
I plan to put insulating covers over the positive terminals, but haven't done so yet.
IMG_8024 by
Lawrence Alexander, on Flickr