<< That entirely depends on your experience assembling/troubleshooting and the kind of system you want. >>
In addition, assuming you are experienced enough to build/troubleshoot and the person financing the computer thinks the same as well:
[1] COST - if you can build a system with better parts at a lower price, then do so.
The only advantages I can think of in buying a DELL system is the warranty/support and it's a known name brand system - the parts have been designed and tested to work together more reliably than a home made jobber. In order to get low costs computer components, you'll probably use an online retailer or a local small computer outlet. When you buy parts from an on-line retailer, you have to send the part back and wait for the replacement to arrive - which on average take at least a week. That's computer downtime. If you buy from a local computer shop, there isn't a guarantee the part will be in if it needs to be replaced. And, depending upon what kind of warranty you buy from DELL, most, if not all small computer shops only warrant their parts for 1 year. DELL offers 3 years warranty.