Tape storage is, of necessity, a sequential medium. You cannot get to an arbitrary spot on the tape to access its data without winding through all the tape previous to it.
If your backup dataset was originally a copy of everything you are backing up - say, an entire hard drive - and you want to restore just the files in one folder, the restore software has to read through most of the file listings on the tape record until it comes to the specific folder you are looking for, then restore from there. If, instead, you backed up all the major folder as separate datasets but on the same tape, one after another, the restore software still has to skim through the tape looking for the dataset that contains the folders you want.
A hard drive, of course, does not have these time delays in finding data because it is not a sequential device. However, until a decade or so ago, HDD's were a lot more expensive than tape per gigabyte of storage. That probably is no longer true.
To compare pricing, put together the two complete systems "on paper". For tape you will need a drive (and mounting case or internal, and connection cabling) and a significant number of tapes. To do your first complete backup you can calculate the number of tapes needed to back up say, 400 GB, if that's the space currently occupied on your HDD. After that if you practice incremental backups you can calculate how many additional tapes you need for them, up to the point where you start the cycle over again. Now, if you want to have TWO backup copies (one at least stored off-site), double that. Add a few spares in case you run into a bad tape. Put that all together and that's your initial investment in the system.
An important factor today is who will change the tapes, and how? HDD's are so big it is normal that a tape backup system will require use of several tapes for one HDD. So either you have to be there, waiting to change tapes in the drive, or you need to buy a tape drive with an automatic changer and capacity of (you figure out how many) tapes.
For hard drives instead, choose one or more external hard drives a little larger than your machine's HDD. If your main machine has more than one HDD, consider whether your backup unit should be sized to allow making backups of ALL your machine's drive to one external. Also consider again whether you will do a scheme of full backup plus several incrementals, and how many external drives you need for that. Now, if you're still going with duplicate backups and off-site storage, double your external hardware design.
By the way, many people find they can save money and get exactly the hardware they want by buying an external case, hard drives, and backup software separately and building their own system. If you do that, consider also cases that allow mounting two or more drives in the same case. Pay attention, too, to the type of interface between the external enclosure and the computer, because they differ a lot in speed and that will be important for long jobs copying a LOT of data. Others are much more comfortable with a complete package from one source. But I suspect among these, "pocket drives" built for use with laptops may be the most expensive option.
Another option to consider, whether for all of the system or just for the second off-site copy, is backing up via the internet to a remote backup service.
Once you put on paper all the components of each system - tape versus external hard drives, maybe also internet services - you can compare costs. My guess is the external HDD option won't be much more expensive. The difference may be worth it for the speed and convenience of making backups and restoring via HDD compared to tape.