Both are excellent calibers. Factory ammo should be available in any gunshop. 243 might be a bit more common at the 'Marts'....
Which one? Depends what you want it for.
I'd get a 243 if I was going to shoot mostly varmints, and the occasional deer or antelope.
If I was going to hunt deer and antelope, with an occasional varmint thrown in, 25-06.
“May we never confuse honest dissent with disloyal subversion.”
Dwight D. Eisenhower
"If the freedom of speech is taken away, then dumb and silent we may be led, like sheep to the slaughter"