I love having my two screens set up on my desktop and while I lose all that with my laptop the convenience is something else again. Just loving being able to take it away for the weekend and catch up on stuff and not be stranded. My ipad is great for things like dp but sometimes you just have to code!
Desktops are better for most reasons. Better cooling and you can update most of the hardware when needed. Laptops are good for mobility. If you want a laptop, pick one that will last a long time and can easily upgrade.
Depends on budged. You can get a faster PC for less money with desktops, while comparatively identicaly speced laptops are more expensive. One think you have to understand is that forum posting, multiple tabs etc. not only requires point&click (which you can do just as well with a mouse and an external keyboard to a laptop), it requires comfort. And laptop allows you comfort, allows you to feel more social, and even more healthy. I would sugest you get a (used) laptop. It don't matter if it's been used for a bit, newer versions don't age badly. Plus, it wil save you a lot of money on tissue paper, comfort, health bills for prolonged sittings etc.
Desktop for routine, mutlitasking, for typing etc. I.e. Desktop for work related activities. Laptop for leasure. Sure, you can "work" on a airbook, it's just not cut for all the productivity of a dual monitor and multiple tasks at once. So, laptop for me does rank high in mobile formats. (Is my go to because it can communicate at least on equal footing with desktop documents.) I would go for a tablet but the legal issues with the office package make me want to shun it down.