They are equal in costs. And its a wysiwyg thing as per rulebook. But, it is not stated anywhere that you need to use the stuff that comes with the sprues."Power axe", "power lance" and "power sword" are now completely different and that is why on page 61 of the rulebook they have different entries.
And there are units for armies that contain possiblities for certain different options..
For example, my Imperial Guard sergeants (and few platoon / company commanders) have been using wysiwyg powermauls and -axes for ages now..
Imo there is no point crying if a Imperial Guard Sergeant of Officer chooces to wield a maul of axe (or even spear) instead of sword..
Not unless it is explicitly stated in a faq that you cannot choose to glue weapons for models other than the ones that come in with the sprues with the said unit.