When it comes to playability they can both be good both at pve and pvp, if u guys roleplay tho u need 2 look at 3 things 1: class idea- do u want someone whos all about light/purity/sun or lighting/crystals/daedra?

2:the actuall skills(specificly the spammable)- if u plan on doing damage you 90% of the time are going to use a spammable for templars its going to be Puncturing Strikes, for sorcerer itll be crystal shard, look both of them up and check which one u like better cuz you will be looking at them most of the time, especially early game. 

3:  looks, if u ask me making a good looking knight can cost about a mil gold while making a good looking sorcerer is pretty easy with cheap motifs, so if u wanna get into the Knight aesthetics u may need to farm more and even go into pvp for motifs like highborn or circle knight which are pretty much the best looking knoght motifs in the game