I'm building an on portfolio view and there are two ways to show your value.

a. Take the price of 1 unit of token multiplied by the amount of tokens without slippage
b. Take all of your tokens, run them through a DEX and show your value with slippage

Which one is better?

b seems clearly better all else equal. But it's also going to more complex and error prone, and depending on the size of your target user's holdings, you might be taking on a significant speed/reliability hit for a very minor accuracy improvement.