First, what will be the purpose of your desktop? Is it for work or games? Next, how much are you planning to spend??

Branded computers are a bit pricey and you will have limited room for upgrading it.
Assembled computers are generally cheap since you will have the option to choose which part to purchase. However you will need someone to help you out with making your rig.

My advice will be to go with assembled computers. Checkout the computers and tech section. You might find some great deals to your liking.