[Shinto]

Any crafted tin weapon is upgradeable, and in theory anything above tin also is.

The main differences are speed and damage type. Smaller weapons tend to be faster, adding DPS. Some class abilities need a particular damage type, like Rogues needing piercing weapons (dagger). It really is specific to your style, though. If you have abilities that do damage based on a % of your weapon, high damage might be better than high speed.

If you are specifically talking about crafted weapons, there are certain effects that can only go on certain weapon types. For example, I have so far only been able to put the effect "Minor Combustion" on spears. Also, some enhancements work differently on different weapons. A mace might get healing focus from the same catalyst that gives nuke focus to a sword.

Another thing that I have noticed about crafted weapons is that larger weapons tend to get bigger increases from enhancements. I suspect this is meant to reflect the fact that they take more materials, and to balance the effect of dual-wielding vs 2-handers.