While I think the Serfas is the best of the bunch, I think it's probably not enough better than the Hotshot to justify the price. I see where they're coming from - the Serfas is built very heavy and has excellent optics and a bigger battery and is brighter, but the Hotshot hits a sweet spot of waterproof, pretty durable, still very bright and a great price.
I'm not sorry to have spent the money on the Serfas, but unless you're really concerned that your taillight be able to stand up to being run over without damage, or if you absolutely need to have the brightest < $100 light you can get in a small package, the Hotshot is probably the better choice.
__________________
Work: the 8 hours that separates bike rides.