If you are looking to buy a new Mac and and wondering wether to go for a laptop or desktop there are few questions which you need to ask to help you decide which one.
Do you work solely from a desk/ office or do you require a mac that is mobile. The second question is performance. If your not that bothered about performance then a laptop might be the better option, giving your the flexibly of working from a desk or not. The final question would be your budget? If you are on a budget and your concerned about performance then you might want to consider a desktop machine.
After answering those questions you are in a better position to make a decision on which machine to go for Laptop or Desktop.
Laptop:
Advantages are clearly weight, mobility and flexibility to work in most environments. You can also add a external monitor to your laptop when its plugged in and sitting on a desk in an office. As for 
Monitor range well their are loads out there. Ideally the connection you need to look at would be HDMI or DVI. I would certainly not look at a any monitor that connects by VGA. With everything being digital and High Definition these days a VGA connection really is a step down.
Disadvantage are really performance compared to a Desktop Machine and the max screen size is a 15″ which compared to a 5K iMac is rather small even though the laptop is a Retina screen. As mentioned above you can plug in a external monitor to help with the screen size. You can run everything from the laptop including the keyboard and track pad however to make it easy I would 
look at investing in a wireless keyboard and mouse. The only cables you would need to plug in would be you power cable and the monitor.
(picture above MacBook Air connected to a Apple display which Apple have pulled, you might pick up a second hand one somewhere but a very expensive monitor)
Desktop:
Advantages are simple the performance of the machine you get a lot more for your money with a desktop machine to that of a laptop. You would not have to bother with purchasing additions items such as a keyboard and mouse and monitor (unless you buy a mac min or the mac pro, my assumption is a desktop would be a iMac). 
There is also the Hughe advantage of the screen size and if you went for a 5K iMac well very nice screen to work with. You can add screens to the desktop machine so you could end up with multiple screens I have seen 4 screens attached to one iMac. (bit over the top in my view but possible)
Disadvantages are clearly the obviously one with mobility or lack of it. The machine would be fixed in place. I have already referred to it would be if you went down the road of the Mac Pro or Mac Mini they would still require a monitor and keyboard, mouse to get them up and running to that of a iMac. 
However looking at the Mac Pro you are dealing with a very high end machine for a specials job like video editing. Looking at the spec’s of the machines available around Feb 2017 there is not much difference in performance with a
5K iMac and the high end Mac Pro. I suppose it is down to preference.
In Short as I eluded above the question you need to ask is do I require a static or mobile machine. The mobile machine can become a static machine where as the Desktop is a desktop and lugging that on site would not be practical. I suppose its why a lot of business and private users always go for the laptop because of the massive advantage it has over a desktop mobility.