I asked myself the same question. Here are the 2 main differences:
1) As Ninchenl already mentioned, the T5 model has an additional (!) cable (USB-C to USB-C), the T3 model only has the USB-C to USB-A cable.
2) Perhaps a more important difference is the USB-C port: the T5 model has the USB 3.1 Gen 2 (10Gbps) and the T3 model USB 3.1 Gen 1 (5Gbps) interface. The transfer speeds in the brackets are, of course, maximum speeds. Samsung gives these figures: 540 MB/s (T5) and 450 MB/s (T3) read and not write speeds. According to external tests, these figures are very close. So the difference is about 10-20%.
If you really want to know the details (keyword: 64-layer TLC V-NAND), look here:
https://www.anandtech.com/show...
Now the question is whether a price difference of 25.- (500GB model) is worth it. It should also be noted that your laptop/tower etc. also needs a new USB 3.1 Gen 2 port in order to use the increased speed of the T5.