I want to offer a counterpoint to B, which is that a market with a clear leader is a market that isn't nearly as big as you think it is. The reason for this is that peeling customers off an incumbent is a lot more difficult than it seems at first. The temptation is to tell yourself, "This is a success if I get even a little bit of the market." That may be true for a market that's wide open, or that doesn't have a clear market leader. But if there is one, then your market size is really more like 20% of the addressable market, and your little slice of success has to be a lot larger than you think it is. And if its a good market, you've probably got a LOT of competitors thinking the same thing, and you're fighting it out over the slice of the market you can actually get to. I've experienced this in the event registration space, where a few companies like EventBrite dominate, and 300 other ticket companies fight it out for the 10% of the market that's actually up for grabs. Your marketing costs skyrocket, the customer acquisition costs are dramatically increased, and you're looking at a long slog to get to profitability.
All of that said, this doesn't mean don't do it. I'm a big fan of niche products, and being able to do the technical side means you've got an advantage over some potential competition who have to hire or contract out that part. But be aware that business success in a venture like this is harder than it may seem at first.