This is one of those things that people feel pretty strongly about. We sit firmly divided on the issue, with one of us preferring the round toe, and the other the split toe.
The round toe bootie is the classic design, just like every other kind of footwear pretty much, there is a single large toe box. These are probably the most common types of wetsuit boot, and certainly the pricepoint models will feature round toes.
Split toe designs come as either a completely split toe box for your big toe, or as an internal split which means from the outside it looks like a round toe design, but there is an internal wall between your big toe and the rest of your toes.
The benefit of the split toe is that some people find their feet rolling around over the edge of the classic round toe boot. As such the split toe design prevents the foot from rolling inside the boot, therefore providing improved grip and precision.
For many of us, split toe designs are simply uncomfortable. Having things between your toes can rub, or just feel weird, which negates any potential benefits. It could also be argued that well designed round toe boots dont roll.
The good news is that both are widely available and similarly priced. You can make your own mind up on which wetsuit boots you prefer.