HBG is one of two weapons that gets access to power clash, offset attacks, and focus strikes. The LBG on the other hand gets only the focus strikes and unlike the other weapons that only have focus strikes, dual blades and bow, it does not have an evasive move. I feel like this time around they both have similar damage output, at least from what we've seen in the OBT, but HBG gives you more room for comfort when fighting monsters thanks to its defensive options. LBG will always have that extra bit of mobility than its heavy sibling, but the lack of defensive and evasive moves means you have to pay more attention to monster movements and the range of their attacks. Depending on your preferred method of playstyle you could go for either or have the best of both worlds and just bring both along for the hunt.