I suggest go for brand desktop unless you're familiar in DIY because:
1. More reliable - they should conduct burn in test and inspected by QA all the parts function properly before exfactory.
2. Bundle softwares - besides the original OS (major is Windows right now), some of brand will bundle application softwares for paper work, photo touch up, etc.
3. Customer service and warranty -
Frankly speaking, don't think about the flexbility of "upgrading" later on. As the growth technology, you'll find the cost of upgrading is close to buy a new one after a certain of period.
Although I'm work in Electronics technical field, I still purchase the brand PC, to save my time for trouble shooting and with above reasons.