Are alternate (more costly) weapons always more effective than base loadout weapons?

It depends on the item. Generally you can't "buy power" by buying other weapons, since weapons suit other situations better or you prefer one weapon over the other. They are not straight-up more "powerful", even if it would seem like by looking at the stats, they mostly require more skill/accuracy in use or just fit better in certain situations.

For example, the new MIRV-Grenade for the Raider deals a lot more damage than the EMP-Grenade if all the shrapnel hits. But it's hard to make the grenade hit for its maximal damage. The EMP-Grenade has a utility effect by draining energy from enemy's making them "immobile". On the other hand the MIRV-Grenade can cover an area over an bigger time (first explosion -> shrapnel explosion).

In some cases the buy-able Weapon isn't a huge change in the usage/effect (e.g. the MIRV-Launcher for the Juggernaut or the new Sub-Machinegun for the Raider). In these cases it's up to you what you prefer.

So, you have to choose your loadout based on your preferences and the role you want to play.

To address your question: If someone gains advantage by buying stuff with gold, it simply depends on the players. But, given same skill-level, you only get situational advantages.

I think you can compare the gold from Tribes: Ascend with Riot points in League of Legends; it speeds up your progression. But you won't dominate in League of Legends by simply having a champion the other players do not.

I hope you get my point. :)

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.