Okay, so I figured it out. Here's what I did.
I created 2 new calculated fields, "MTD" and "YTD". Below are the contents.
if month([Date]) = [Month #] then [Date] end
if month([Date]) <= [Month #] then [Date] end
And a 3rd calculation called "View By" which looks like this.
case [Sum by]
when "YTD" then [YTD]
when "MTD" then [MTD]
"View By" is then dragged to the filters shelf. With the same worksheet I now have 2 user parameters, [Month #] and [Sum by]. Now the parameters have full control over the filter!
Users can select the month they'd like to view and what kind of aggregation they'd like. I needed to make this dead simple because most, if not all, users will be unfamiliar with Tableau.
Hope this helps anyone with similar issues.