Since I've been working with both Drupal and (more recently) CivicSpace, I thought it might useful to post some thoughts on the differences between the two, to make it easier for someone new to CMS to make a decision on which one to use.
When we decided to make the move to Drupal, we'd already played around with the system on a couple of dummy installations, just to get an idea of what it was like to work with. We assumed that we should go with Drupal instead of CivicSpace, since we thought of CivicSpace as being Drupal + a collection of modules...we figured we could easily add in the modules we wanted later.
CiviCRM:
This was our first mistake: Some of those modules aren't easy to configure. I'm referring here to CiviCRM, an excellent constituent relationship manager (CRM) module developed for Drupal which is installed and configured automatically as part of CivicSpace. This is a big plus, as I couldn't get CiviCRM to work as a modular add-on to a standard Drupal installation even after a couple of hours of messing around with it. Though this might not seem like a long time to try, and I could undoubtedly have figured it out with more effort, I decided that this wasn't a configuration process a newbie website designer would be able to master.
CivicSpace, on the other hand, installs those modules with ease. Though I did have problems a couple of times with CiviCRM not working properly, the automated CivicSpace Installation worked perfectly, for the most part. It also installed and configured CiviCRM without any technical work on my part at all, other than one question on whether I wanted to set CiviCRM to run from its own database (the instructions said to leave those fields blank to let it run from the same database as my CivicSpace installation, which was what I chose to do).
Though the ability to CiviCRM, especially its ability to map coordinates through the Google Maps API, was a major deciding factor in my choice to use CivicSpace for future sites instead of Drupal, a few other considerations also deserve mention.
Ease of Install: Drupal is listed in the Installatron scripts for our preferred hosting providor. This means that I can install Drupal on a domain by simply logging in to my DirectAdmin panel, clicking on Installatron, clicking on Drupal, and then telling it where I want the installation to go. It does everything for me, even creating and setting up the database, without any further effort on my part.
CivicSpace, on the other hand, requires a little more work, but is also easy enough for any newbie website developer to master (documented here: the automated CivicSpace Install). However, if CiviCRM isn't important to you, Drupal may be a better choice, simply because it comes as a standard installation option for many open-source-friendly hosting providers.
Installation Issues:
While the built-in Installatron installer script worked fine for Drupal, CivicSpace's automated install had a few requirements. First of all, it required safe-mode turned off for the domain. This is a plus, because Drupal doesn't tell you about this in advance...you only find out about it later when your site starts giving you safe-mode errors. Good thinking on the part of the CivicSpace developers to include this in the installation process.
Secondly, CivicSpace strongly suggests that you increase the php / MySQL memory limit in Apache to 24Mb. Some of the servers I worked with had this set to 16Mb, which means opening a helpdesk ticket with my hosting company and asking them to increase it for me, pretty please. They did this for me quickly (and for free) whenever I asked, but if you have less responsive hosting arrangements, this could be a significant obstacle. Many shared hosting environments set their limit to 32Mb, though, so it may not be an issue.
Lastly, CivicSpace gave me a list of file permissions I had to manually change before running the installation, and then after the installation had proceeded successfully, listed a few of those files to change back to more secure permission levels. Drupal's Installatron script doesn't do this, and (like the safe-mode issue) simply causes problems later on if you don't have the right permissions set. You normally see these when you visit your Admin -> Settings page for the first time, when Drupal often tells me that my /tmp/ and / or /files/ directories have the wrong permissions, can't be written to, or don't exist. Creating or changing the permissions on these manually fixes the problem. Like the safe-mode error, this is more of a trivial pain than a reason to choose one over the other, though novice users may find this more of a pain than I did.
Bulkiness/Server Load:
There's no getting around it...CivicSpace is a much more powerful package than the default Drupal core. It's preconfigured to do much more, but at a certain cost...it's much more bulky. There are many, many more modules installed in the CivicSpace installation than in a standard Drupal install. Some of these, like TinyME, are modules I would have installed anyway, because their functionality is very, very useful to have, but it's inevitable that there will be several modules I'm never going to need. This means a much greater amount of unused code sitting on my webserver with a CivicSpace install, and a greater number of processes running on the server, which translates to greater server load.
Also, the administration of CivicSpace just "feels" more bulky and ungainly...Drupal, in comparison, feels much more clean and flexible. If you don't need CiviCRM, I'd highly recommend installing Drupal and figuring out how to install the modules you'd like to add instead of a full-blown CivicSpace installation.
Conclusions:
In the end, the choice is up to you. Whichever you choose, I'm certain that it will server your organization far better than a competing product (Mambo, PHP-Nuke, etc). If my comparisons above seem trivial, it's because (for the most part) they are. The two systems are growing very close to each other, and this is one of their greatest strengths, since their development, support, and feedback processes benefit greatly from each other's communities.
