There is a huge amount of difference. In short i would just recommend getting any sets by DuMonde, Flutlicht, SHOKK etc. for Hard Trance... sets by Kai Tracid, Hennes & Cold, Scot Project for even Harder Trance, and then for Hardstyle check out sets by Dana, Pavo, Luna, etc. for Hardstyle just try and get some of the Qlimax sets from 2003. That shud explain it to ya.

The way I see it, hardstyle is just a sub-genre of hard trance. Some will argue otherwise, saying it has more in common with hardcore than with hard trance. (What people on this board consider hard trance often isn't really all that hard at all.) The differences are often pretty obvious, and other times not so obvious. Hard trance is usually much more melodic and more complex than hardstyle. As mentioned check out SHOKK, Scot Project and Kai Tracid. Unfortunately a lot of the big hard trance DJs are going pretty commercial lately. Hardstyle is often ultra-simplified, with hard-as-fuck kicks and bass. Check out productions by Zero Vision (wicked!), Technoboy, Deepack and Thomas Trouble, as well as the DJs mentioned previously. There can be a lot of grey areas between the two though. You get people like Uberdruck and sometimes Blutonium Boy, who's productions sound a little more like hardstyle based on the sounds they use, but arrangements and song structures are a little closer to hard trance.

If it were up to me, I would consider all of the above to be part of the hard trance genre.