If you take a look at the table here, you'll find that when comparing Radiant to Fire, Fire is more resisted/Immune (37 with resistance, 40 with immunity) than Radiant (Resist 4, Immune 0). However, there are 9 creatures vulnerable to fire and only 1 (apparently) vulnerable to radiant.
Those statistics are, of course, for the entire MM, and unless you plan on using an even distribution of monsters from the manual in your campaign, you may find that fire or radiant is more effective for the player. Also keep in mind that there are creatures that REQUIRE fire (or acid, as the following example shows) in order to be killed, a.k.a., the Troll (MM.
Switching damage types is a great way to add flavor to a game/character. If you follow the guidelines listed in the DMG pg. 283/284, you should be fine. Here they are, in case you can't get to your copy (you'll notice they don't say not to switch damage type, nor do they balance by damage type, as it seems like it's generally not considered to be a relevant factor)
Creating A Spell
When creating a new spell, use existing spells as guidelines. Here are some things to consider:
If a spell is so good that a caster would want to use it all the time, it might be too powerful for its level.
A long duration or large area can make up for a lesser effect, depending on the spell.
Avoid spells that have very limited use, such as one that works only against good dragons. Though such a spell could exist in the world, few characters will bother to learn or prepare it unless they know in advance that doing so will be worthwhile.
Make sure the spell fits with the identity of the class. Wizards and sorcerers don't typically have access to healing spells, for example, and adding a healing spell to the wizard class list would step on the cleric's turf.
Spell Damage
For any spell that deals damage, use the Spell Damage table to determine approximately how much damage is appropriate given
the spell's level. The table assumes the spell deals half damage on a
successful saving throw or a missed attack. If your spell doesn't deal
damage on a successful save, you can increase the damage by 25
percent.
You can use different damage dice than the ones in the table, provided
that the average result is about the same. Doing so can add a little
variety to the spell. For example, you could change a cantrip's damage
from 1d10 (average 5.5) to 2d4 (average 5), reducing the maximum
damage and making an average result more likely.
\begin{array}{c|rr}
\text{Spell Level} & \text{One Target} & \text{Multiple Targets} \\
\hline
\text{Cantrip} & 1\text{d}10 & 1\text{d}6 \\
1\text{st} & 2\text{d}10 & 2\text{d}6 \\
2\text{nd} & 3\text{d}10 & 4\text{d}6 \\
3\text{rd} & 5\text{d}10 & 6\text{d}6 \\
4\text{th} & 6\text{d}10 & 7\text{d}6 \\
5\text{th} & 8\text{d}10 & 8\text{d}6 \\
6\text{th} & 10\text{d}10 & 11\text{d}6 \\
7\text{th} & 11\text{d}10 & 12\text{d}6 \\
8\text{th} & 12\text{d}10 & 13\text{d}6 \\
9\text{th} & 15\text{d}10 & 14\text{d}6 \\
\end{array}
