I don't believe that desktops will die easily if you go run it 24/7. I believe it dies much faster when you forcefully turn it on and off repeatedly due to power trips/surge. Heat is also another enemy. Therefore I always believe that a proper ventilated desktop will always last much longer than a laptop because all laptop has limited space for cooling .

My company has a bunch of desktops running as enterprise software license servers in a data center for at least 6~7 years 24/7, never turned off and they are still alive. Core2Duo E6300, 3GB RAM running on Windows Server 2003. Never even changed a motherboard or hard disk. All of them are running RAID-1 as an "insurance policy" though. Company is stingy and refuse to buy proper rack servers because it's under utilize and a lot of software need to plug in usb license dongle which cannot support Virtual Machine .

TS , my advice is, if you want something to last very long time . Avoid a laptop. Not only it is difficult to get spare parts compare to desktops, it has more maintenance cost to cover. Most of the time, battery and the AC adaptor will fail after 1 year which your warranty has ended. Then comes your motherboard and the lcd screen. These are common problems with laptops.

Only if you must have the portability of the laptop, then by all means go buy it. Even so, getting at least 3 years warranty is a must.