Assembled computers are far more cheaper and you get to customize the hardware according to you needs. Branded computers on the other hand are expensive with no option for selecting the hardware.
Branded computers include incompatible hardware
eg) A Pentium P4(800MHz FSB) is paired with a 233 Mhz DDR Ram. Thus the P4 is forced to run at 233 Mhz
Although this problem of incompatible hardware is also there in assembled computers but here the customer has the option to choose his hardware.
I could go on and on .
If you have the knowldge about computer hardware go for assembled computers or else go for Branded computers of reputed comapnies where you can get good customer service.