I own both rifles. The model 760 was purchased in 1975 and the 742 was purchased in the early 80's both have been shot quite a bit with no problems.
But as stated by others keeping em clean and oiled is the key to long, trouble free service
If you aren't much of a regular shooter keep the .243. I love the cartridge. And it sounds like you'll maintain it well after everyone's admonishments. 'course my 760 is a .308 -- lol.
Originally I was going to say "let your BROTHER pick first."