If I were doing this with drives, I'd grab an 8x SATA card, 8 4TB drives, an external hot-swap shelf, and setup RAIDZ-2 with compression on it.
Can somebody compare the cost for tape?
You pay about ~$1800-$2500 for tapes, a tape driver, and SAS card, depending on vendor, an extra 27 tapes to store 20TB again would be about $700. The big win is that the tape setup take less space, changing tapes every 800GB is not so hot though, but you get used to it. I've used 4 tape drives in parallel to be able to backup all data generated durng the day, it's ok, you learn to handle that with the morning coffee.