Hi Clem,
Several factors swung it. I'm building the boat for us to use so I wanted confidence that I would enjoy sailing the finished boat.
We couldn't test a Storm 17 in October when I first got interested in building a Swallow Boat, but Matt kindly took us out in his BR17.
Water ballast was a big plus for my wife who isn't keen on unballasted boats (we've previously had various bigger boats up to 32 ft).
The transom design means that there is a lot more beam carried for more of the length and so the BR17 looks a lot bigger and more stable (though in reality it is only a few inches longer than the Storm 17).
The outboard well is a much better design, located on the centreline, forward of the rudder.
Thanks to the foam buoyancy in ballast tank (see Julian's post on my BR17 build thread) the cockpit sole is always above the waterline, even with the ballast tank flooded. It's self-draining rather than self-bailing.
The scarf joints on the major planks look like a really hard job on the Storm. On the BR17 you get finger joints instead, which I think are less daunting, but not necessarily quicker due to all the sanding to make a snug fit. See my "first finger joint" thread and you'll see what I mean.
The major downsides of BR17 vs Storm17 are cost (bigger boat plus false floor => more materials) and increased build time. And the fact that there aren't many home builders out there to share design-specific tips (though Julian from Denman Marine in Australia has been a huge help so far.)
I hope this helps in your decision.
Anthony