Every now and then a debate erupts within the SharePoint and Drupal communities about the other platform. See for example this recent SharePoint community forum post and this reasonably balanced Drupal community discussion.

My take is that Drupal and SharePoint are really very different systems, though I can see why people compare them.

What's Similar

Both SharePoint and Drupal are "platforms" that require substantial developer intervention and savvy to not screw up. Both can boast extraordinarily vibrant communities, especially developer and integrator communities, who tend to noisily hype their solutions.  Neither platform is particularly beloved by end-users. Both platforms have a history of difficult upgrades.

Both Microsoft and Acquia are aggressively promoting fully-managed, cloud-based versions of their solutions.

What's Different

Let's start with technology. In addition to the obvious distinction between their PHP and .NET language bases, there are some broader architectural differences.

• Drupal has done a better job of creating a core/module architecture to support rapid innovation, if much confusion as well. SharePoint's "app model" is new and comparatively untested.
• SharePoint does a much better job of playing with other applications. Or maybe it's more accurate to say: other vendors are highly motivated to create connector web parts. Drupal tries to be quite self-contained, sometimes in a very parochial way, and the Drupal community has only recently put meaningful effort into connectors and serious integration frameworks.

Of course you'd also want to consider licensing and support model differences, though I think these tend to get overstated. In our experience, against the same set of specifications, your longterm cost of ownership will not likely differ much between SharePoint and Drupal. You may see comments accumulate below that vehemently disagree.

The biggest difference really revolves around intent.

• Drupal is a combination Web CMS + Community platform that over time has really come to focus on public-facing scenarios, though some customers use it for digital workplaces.
• SharePoint is an omnibus information management and portal platform deployed primarily for internal scenarios, though some customers use it for public websites (we'd actually discourage that for larger enterprises).

So, you would want to consider Drupal versus SharePoint head-to-head in only a narrow set of use-cases. In our WCM and Social Collaboration vendor evaluations the two platforms score quite differently from a scenario perspective. If you're an RSG subscriber, you can visualize this delta via our Custom ShortList Builder.

In other words, take a business perspective first. And when you do, you may find that SharePoint versus Drupal is actually a false choice...