Let me make the case for when to use an XLR microphone. When suggesting podcast setups, I'll always point to an XLR setup. Here's why.
QUALITY & QUANTITY
First of all, an XLR microphone is a mic that uses an XLR connection. XLR stands for: X connector, Locking Connector, Rubber Boot. But most importantly, XLR is the industry standard for high-quality audio inputs. You'll also find that there's a much bigger pool of options for XLR microphones as opposed to USB mics. Most quality XLR mics will be in the $100 range, with budget mics as low as $48, and high quality mics all the way up to $600 plus. However, a $100 microphone is going to be great in quality. Listen to the $99 Rode PodMic on episode one of our podcast, and the $100 Sennheiser e835 on episode four of our podcast.
PORTABILITY & DURABILITY
You'll find that most XLR microphones are very portable, in that they can easily fit in a backpack or a laptop bag. I used to carry around a small backpack with 4 Sennheiser e835 mics, 4 XLR cables and an audio interface. That made for portability and quick setup when going on-location to record an episode. The build on most XLR mics is solid as well, utilizing metal for the mic housing and not that you should drop your mics, but they often hold up well after having been dropped.
If you're a solo podcaster, then a USB mic could work well for you as a simple plug and play solution. However, if you're looking to add more hosts or guests to your show, in person, then an XLR microphone allows for scalability for your show. This is because when using an XLR mic, you'll need an audio interface, digital recorder or a mixer in order to use your mic to record audio onto your computer or onto an SD card. Many of these devices are available with multiple inputs, and more inputs means more mics, and that means more people on your show, if that's your goal. It is possible to connect multiple USB mics to your computer, however, it is a rare case to find podcast setups like this. Go for XLR.