If you have a chronograph, you may notice a slight difference in velocity when using Winchester vs Federal brass in .243. If you are 10% below max when working up loads with same (Federal or Winchester - not mixed) you will not have any problems at all. Don't need to go buy one if you already have the other.
If you work up a good load in one brass, and it is near max, don't just switch to the other brass and assume it will be ok. Lower the load a few grains and work up to confirm different brass will like the same load.
Factory brass is a little different by each manufacturer, but in the .243, you will not notice it. If you try using brass that someone has made ie. 7-08 to .243 or 308 to .243, you need to start about 20% lower as they can be a little thicker, therefore less case capacity, and pressure will build quicker.
You might try some H4831 in that .243. My Ruger loves it. My Remington .243 WSSM is still in the works, but all loads have shot moa or less so far, but still working up as no load data.
Good Luck, be safe, follow the rules, and you will be fine.
Steve
P.S. Military Brass and Commercial brass are a LOT different. DO NOT substitute military brass using commercial load data without starting out 15-20% below max loads. You also have to deal with primer pockets, neck thickness, etc.