Yes, it will slow down on USB 2.0 ports. I would guess it would operate at 30 Mb/s read and 10-15 MB/s write. It is important to note that this would be in ideal situations. This occurs when copying a few large files such as movies. When copying many small files the transfer speed will be significantly less.
Its not necessarily better or worse. If you think that at some point (while you are still using the flash drive) you will upgrade to a computer with USB 3.0 then it might be worth it to purchase it. If you don't think that will happen then it doesn't really matter whether you buy a USB 2.0 or 3.0 flash drive. Just go with the cheapest one of your desired capacity from a reputable manufacturer.