I realize that Unity is a game engine and XNA is just a framework, but it seems like either platform could potentially work for this purpose.
Which platform has more support in the regard of built-in or community built code/open source projects related to RTS games?
I know that there is going to be a lot of scripting/programming involved either way but if I didn’t have to worry about making my own fast and efficient collision detection, pathfinding algorithms, client/server networking (the more nitty gritty stuff), it would greatly speed up development and allow me to concentrate more on the gameplay and design.
If you really don’t want to be messing with the nitty-gritty as you just said, use an engine.
Even though a lot of stuff is high-level in XNA compared to many other frameworks, you still are ultimately responsible for the code your game is running – down to the smallest nitty-gritty detail.
This really goes for any genre by the way.
However, people can write engines using XNA that attempt to take care of the nitty-gritty for you, just as something like Unity would.
In the way of Unity, since it’s already an engine you can’t really write one in it – but people still write helpful scripts that you can make use of. Doing a bit of poking around on UnityAnswers, there are plenty written with RTS’s in mind: It’d be your job to integrate the ones you need within your Unity game.
So honestly it comes down to which option provides more/better resources for your purposes; hope I’ve gotten you started at least.