Comscore Ad Metrix provides the best instance of measuring the digital landscape at scale. They have a huge amount of the internet crawled with focusing on just this case.
Ad Metrix is well suited for teams that are directly tied to sales teams as well as teams that use DMP and other platforms to help target and retarget specific users for ads or site visits/views.
It may not be as useful if the organization is not targeting specific users or isn't interested in their consumption habits and what competitors are focusing on.
If you can control how data can be used for marketing and knowing who to cater to your content, too, this is super useful. If there's not much, you can do WITH the data you're provided, and it'd be of no use. It's a must if you need to dig deeper into your audience, though.
Site Analytics data - ComScore puts all of the stats you need in an easy to read dashboard. I don't need to search around like I do in Google Analytics to find out who is visiting my client's site, where they are browsing, how long they are engaging with the content. It's also really easy to integrate conversion data alongside website analytics. The webpage heat maps are also an awesome way to look at where people are engaging with the client's webpages.
Infographics, charts - I love how easy it is to pull a client ready chart that I can insert into a presentation for the client to show various stats pertaining to their campaigns (e.g. most visited pages, most viewed videos, MoM stats on orders/revenue/average order value.
Report Builder - Makes it simple to pull reports on site analytics that I can easily add filters to. I can get all the data I need in one report. I do a lot of presentation building for the clients on a monthly basis. This makes it more efficient for me to pull out sharable insights.
The staff is available pretty quickly for questions, but are not really around in-person to meet with me and my team. They are able to solve my problems online and do work sessions as needed via skype.
They're both fairly similar in relaying data for us on our viewers. We've used comScore longer and are able to compare specific data to one another as well as competitor sites
Quantcast's advantage over other tools is that it is 1) free (as is Google Analytics), but that it is a more easy-to-use and basic tool than Google Analytics. Quantcast does not provide the depth of analytics of other tools like Google Analytics or SimilarWeb, but it provides a good starting point for someone looking to venture into web analytics.
I have not used the tool long enough to determine myself, but my understanding is the team has been able to drive growth and sales significantly through the use of the tool.