Normally your streamer's native capacity should exceed the size of your hard drive, plus some extra space. However, purchasing a smaller streamer can sometimes make sense as well, particularly if you want to backup only those files that were modified and not your whole system.
There are two types of backup: The first approach is to use a new tape for each and every backup, regardless the data volume. After backup, the tape is stored in a secure location. The second method is to continue using a tape until it has almost reached its full capacity. This way is definitely more affordable than getting a new tape for every run, but on the other hand, all backup sets will be lost if something happens to the tape.
I do not recommend choosing a streamer by the type of the media, though many people do that. The most important factor is of course reliability and capacity, followed by performance. A streamer that is capable of storing several dozen gigabytes onto a tape is not worth a straw if it requires too much time to exploit the maximum capacity.
In this respect the Seagate TapeStor USB streamer does not get very good grades. However, you should take into account that this one is an external model and certainly has the best connectivity - thanks to its USB interface. Every computer today has at least one or two ports, so attaching this device should be no problem at all.
In the streamer sector it is quite common to advertise every product at double the native capacity, assuming a data compression ratio of 2:1. Actually, it is quite difficult to reach such a good compression, as many types of data are either already compressed (JPEG graphics files for example) or simply cannot be compressed very well.
Before you buy a product, make sure that there is no misunderstanding with regard to the native capacity. Even transfer speeds are sometimes based on compressed data, so better double-check this first.