My preference for singing audio has always been the Shure SM58. The AT2020 will certainly do the job but I'm not sold on condensers being the ideal mics for singing. The only problem is that you're stuck with condensers if you're going to also stick to USB microphones. You have limited options in the Blue Snowball(trash), Yeti (good but overpriced), Samson C01U (well priced USB mic), AT2020 (USB model is expensive compared to XLR) and a few other lesser known USB mics. If you make the jump to professional recording mics (XLR interface) then your options open way up. You'll need an audio interface, just search XLR USB audio interface on Amazon and you should be able to find one for around $50.
From there you have far more variety in microphones. Shure mics are my favorite, and they have a wide range of prices as well. SM48 for around $50, PG48 for around $60, SM58 for $100.