FortiGuard Web Filtering Service is well-suited for an organisation that is able to funnel all its Internet connectivity through one device, i.e., the office environment - or perhaps where WFH workers are using a remote desktop or VPN. It allows management/IT to minimise risk by blocking websites that may be harmful, as well as websites that may enable data leakage, or even are websites that people ought not to be visiting in the workplace.
Email, web browser, and data storage areas will fulfill the security of mobile devices very well. It provides multiple methods of data filtering and classification to ensure important data can be well protected. Unfortunately, it does not support you to make regex-based definitions. for this reason, you need to work on the rules a lot. Another disadvantage is that it cannot monitor Linux systems.
Some websites are classified as Meaningless content, especially links used in emails to unsubscribe from lists. Luckily FortiGuard Web Filtering Service is responsive about reclassifying those quickly.
Symantec DLP works good for our very simple and low complexity use cases and I would highly recommend it for them. However, I can't speak to highly complex or advanced use cases that might arise in certain industries like finance. I would recommend that if you are seeking an Enterprise DLP solution that you absolutely must explore this product as one of the leaders in the space - not doing so would be negligent in my opinion.
We selected FortiGuard Web Filtering because it came in the service bundle with our FortiGate unit. After testing the service for a couple of months, we disabled the web filtering that was running on our client machines and have not looked back. It's been easy to manage, effective, and fast. There isn't much more you can ask for from a filtering solution.
Symantec DLP is the best. Put it in a head to head competition and it will win. However it can be a bit more expensive than other products so don't compare on price, compare on quality of product.