There are various aspects to note here:
Are these alternate purchasable options always an upgrade
Firstly, there are weapon and item upgrades. These will indeed improve the statistics of that item and give you a clear advantage over other players with the same item, but less upgraded.
Secondly, there are alternative items for the same item-slot (as you mentioned). In the first two big patches that introduced new, alternative weapons these clearly were overpowered, but have been patched (nerfed) since. The future will show if Hi-Rez has learned on how to balance items, or if they are indeed not capable of releasing well-balanced items - be it because of not enough testing, just not bothering, or a calculation on their end on sales. (I’m talking about release-state ofc. As I said, the previous issues have been solved in a relatively short amount of time as well.)
So alternative items will not give you a clear and/or unconditional advantage, provided you compare them on an equal upgrade level.
This also means, with a given, limited amount of currency, comparing upgrading your default item instead of unlocking a new one, you will probably be better off - stats wise, because your newly unlocked item (which is as expensive as several upgrades) will not be upgraded at all.