IMO the machine gun is definitely a better trade than the Spur. You get infinite flight early on with the MG (as long as you keep it at level 3), and the spur's damage is rather unimpressive compared to both the blade and the Nemesis. But that infinite flight is incredibly useful.
Although I do agree that the machine gun is a better trade, I'm going try to defend the Spur
(sort of...)
One of the best perks the Spur has is its level system. You never have to worry about how much exp you lose because you just hold the button to charge it up(gain exp).
The Blade has an extremely high dps, but that's only at an extremely close range, as in, like 2 pixels. At that range it's very easy to get hit and level down. This thing is meant for bosses, not regular enemies. And most bosses you face after getting the blade are too mobile, or require you to move constantly to avoid damage. So you can never really get the full damage potential out of it.
The Nemesis really is for advanced players. Because it sort of forces the player to dodge exp, which can be annoying most of the time. I mean sure, you can just quickly change your weapon. But if get any exp for the weapon, it simply becomes a lvl 3 polar star, and then just plain shit. This forces the player to damage themselves in order to make the weapon useful again. Although I will say that this weapon would be the best for bosses. This wouldn't be the most optimal weapon for use with bosses that have ads that drop exp.
The Machine gun suffers the level problem. Only the level 3 has the flight ability and decent damage. But if you level down, you not only lose the flight ability, but also some damage and range. This weapon I feel, is meant for going through levels more than fighting bosses.The Spur is better in my opinion, for taking out regular enemies. Cause all you need is to charge it to level 2, the shot has good range and damage, along with the ability to pierce enemies. Something that the machine gun doesn't have.
TL;DR
All these weapons have some sort of drawback/niche to them, whereas the Spur doesn't really have any major drawbacks(aside from the charge rate). Although the Spur doesn't excel at anything in paticular compared to the other weapons. It's a viable weapon for any situation, and that's what makes it a good weapon.
I'm not saying that any of the other weapons are bad either.