Well see, the whole idea behind costume slots is that you don't have to look at the armor you're wearing, you can just put on something prettier on top, such as the salamander suit if you prefer it.
Of course, if you like the general aspect of Plate, then that's fine too.

If you plan on using Volcanic Plate armor, you have to understand that you're sacrificing your offensive potential for a stronger defence, through attack speed. It's up to you if the reduction is manageable or not. The reduction in speed may or may not make you more open to being hit.
Volcanic salamander won't reduce your speed, but you'll have less normal defence. Remember that normal type attacks are less common in T3.

~~~~~ Straying from the immediate subject matter ~~~~~

However, if you're comparing 'Volcanic' armors, you should include all of them in the selection.
Volcanic Salamander, Volcanic Demo and Vog Cub (Not technically Volcanic but bear with me) all have the same defensive traits. Same normal, and same elemental defences, along with fire resistance, according to stat bars.

In the end it just depends on what you want as armor bonus.
Volcanic Salamander is a specific slime bonus, so if you don't plan on killing many slimes that's kinda boring.
Volcanic Demo has a charge time reduction for bombs, but if you're not a bomber then that's also null.
Vog Cub, the 'popular' one, is a sword attack speed increase, which is also 'meh' if you're not a swordsman (but the majority of players are).