Math 341 - Operations Research 1, or
Math 409 - Calculus of Variations
Also, which out of the two is more helpful generally that might also be used later in my masters and/or PhD? Can I have a little idea about what I'm going to be studying in these two courses? I have so far taken Cal 1, Cal 2, Advanced Cal, ODE, Real Analysis 1 and 2, Algebra, Probability, Statistics etc. And yes, I plan on doing a senior year project/thesis as well. Thank you in advance.
It depends on what you want to do during your grad years. It really does.
If you want to do OR or Financial Engineering then OR it is.
For a general pure math program you are expected to be able to do a lot of algebra, topology. You are expected to have analysis around your fingertips (Real, Complex, Functional).
For an applied math program it again depends on what you wanna do. Generally incoming class have strong backgrounds in PDEs, Numerical (Analysis, Linear Algebra, ODE and PDEs sometimes Optimizations). Also Modeling Classes, Asymptotic Analysis classes.
In general for math, math programs my PERSONAL suggestions are: PDEs $\simeq$ More Analysis $\geq$ Topology $\geq$ More Algebras $\geq$ Calculus of Variations $\geq$ Operations Research.
Take OR only when you think it will help you with exactly what you want to do. I personally have difficulties believing that it is very useful in getting in a general math program without a focus in OR.
Disclaimer: For people with OR background: I admit that you might have every possible mathematical training that is needed and it is itself a fascinating and difficult subject. However the extent of this answer addresses directly to what a general math program is looking for. Some noted partial exceptions are in the comments below.