That depends; if they're cheaper in hammer cost, I've found MANY times over the past versions of Civilization, Fall from Heaven included, that I'd much rather have the option of building, say, a warrior, and then pay the gold to upgrade him to a swordsman, than to build a swordsman directly. More so if you factor in those blessed events where all of a specific type of unit gain combat 1 automatically; as it does not affect the upgraded versions unless you actually upgrade from it.
Looking over the Civilopedia, I say... First of all, the Radiant Guard costs half as much hammers; being able to build them in half the time can sometimes save your life in a war, or, if you have coin to spare, you can field them that much faster. Furthermore, the radiant guard is melee, and strength 4; Ratha is mounted and strength 5/3. The Radiant Guard is better for defending, and the ability to gain the City Raider promotions can go a long way, if you use them that way.
Edit: If I had my way, personally, I'd have all the units never obsolete, but have a sub-menu for building, so you can just select all melee units, then a specific type, because many reasons more than I just listed can potentially come up.