To summarize;
all in one suck balls they still use laptop hardware that dies quickly
ultrabook is meant for real world busisness/studying and high quality ultrabook is expansive.
Desktop depends, it's what has been put in there.

Off topic,
small form factor desktop will be the future home desk computer we will known.

Take example of the piston Xi3 or the intel NUC, for power efficiency, volume , light weight and portable, and what it can delivers is quite excellent for general purpouse.

I prefer tower desktop, it can be rig to your specification... 

unlike all-in-1 destop and ultrabook, laptops, netbooks it has long life span than this...

Depends on what kind of game...

For Angry Bd on the go, laptop does the job...

Desktop ALWAYS provides superior cooling and access to hardware for upgrade/repair. No battery to overheat and start fire. Lets look at the Mid tower below:
 

 http://www.newegg.com/Product/Product.aspx?Item=N82E16811146058

Of course at this time - this model goes for under 50$ US. This is an excellent case for internal observation and operational efficiency. 2 fans comes with this model as shown. You can add up to 3 more fans for optimal cooling. The movement of heat AWAY from processor and components is the primary reason for the fans and heavy usuage worthy of the top games available.

You will have to pick and choose your own motherboard and processor for the application you desire, but I would strongly recommend that any computer you plan on putting together has a minimum of 4 core processor and motherboard with 4 slots for RAM.  Adding SSD and other peripherals would be up to the builder. I can easily build a computer worth more than 1800$ US just by using my knowledge and less than 700$ US.

Power Supply! - Best to use something in the 750 Watt range to power EVERYTHING you cram in there. (650 Watt range minimum)

VIDEO CARDS! - Here is where things get wild on price and function. Some video cards go for as much as 5000$ US and as little as 40$ US. Usually a video card in the 70$ US to 120$ US will run most games with optimal effectiveness.

The aforementioned 5000$ US video card is used in CGI animation and developement. Although it is a superior video card, it is not as good for gaming as lower priced models. In other words, pure OVERKILL.

Here is what a 5000$ US video card looks like:
 http://www.newegg.com/Product/Product.aspx?Item=N82E16814133494

The Achilles Heel of notebooks/laptops is COOLING. Although some people prefer compact devices, let's be honest here...... if you plan on gaming more than 2 hours at a time, then Laptop/notebook is the wrong choice.

No battery?

Real men have massive backup power banks attached to their home computers and networks.


Nothing like an extra 20 minutes of activity during a blackout. Confusion for all the neighbors who see no lights but still see my wifi networks for hours into a blackout.

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On another note, it is still possible to setup a decent PC with a 400 or 500 watt powersupply. A "650 minimum" is by no means a standard.

Processors have gotten much more TDP efficient in recent years, as well as grahpics cards. Doing much more performance for the power used. 


For example, an i3 or AMD A8/10 APU based desktop can easily fit into 400watts and have 120 watts spare headroom. The AMD A4/6 APU based desktops with 400watt power supplies have even larger.

Fact of the matter is, PSU should be the last item selected in a build. To select a PSU, you should add up the wattage of all the other selected items @ maximum usage. Add ~100 watts to that total and round up to the next wattage class (As they normally are in intervals of 50 watts til 1000 watts)(100 watts will normally cover for Capacitor degrading for up-to 5 years, PSU should be replaced if used for more than 5 years as the capacitors are simply not designed for longer use anymore) if no future upgrades are planned until your next build. If one plans to do upgrades in the future, add the expected max wattage of the type of item wanted in the future to the calculation.

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All-in-one PCs are build on low-end laptop components and put into enclosures that use less cooling than even their laptop counterparts. That should sum of the disaster all-in-one's can be.