If you want longest life, no doubt 24/7 is the best way to get it. Thermal cycling remains the #1 killer of electronics, especially with the heat in a typical fast PC.
But you pay for that in electricity - say £100+ per year per PC.
BTW the latest 3GHz Intel processors draw much less power than a few years ago (judging from the chip temperature), and I have just built a PC with a 128GB SSD (Corsair I think) which cost about £350 and this dramatically improves overall performance; it is like a 10GHz CPU.