During my Win ages I used to have Linux installed in a dual-boot configuration, using Mandrake, Debian (I think it was woody) and Gentoo was my last dual-boot Linux. During that time I removed Win completely from my machine (I kept some VM ;) ) and kept Gentoo as my solely OS. I've made two distro switches since then. I'll describe my experiences for each distro:
Pros:[ul][li]You might learn a lot[/li][li]Updated software[/li][li]Rolling Release[/li][/ul]
Cons:[ul][li]Very Time consuming[/li][/ul]
I have very good feelings towards that distro. It made me learn so much about the whole Linux software stack that I would recommend it to anyone who wants to learn Linux internals. Building form source forces you to check dependencies, choose supported compile options, etc., which makes you acquire the picture of how different programs and libraries are related in a Linux system. It's very well supported through the wiki with all howto's you may need to build your complete system. The problem is that the compile every package approach is very time consuming and you might have to do some real work on your machine other than compiling packages ;) .
Pros:[ul][li]User experience[/li][li]Closed software vendors support[/li][li]Integrated Web services[/li][/ul]
Cons:[ul][li]Six months distribution updates[/li][li]I don't feel very comfortable about some Canonical moves towards Open Source[/li][/ul]
If you don't care so much about the OS but about doing your stuff this is the way to go. I had no big troubles with it and worked fine for me. Nevertheless, as Ubuntu grows I feel like your are trading some of the Open source values and freedom (specially with the new cloud services embedded by default) for comfort and a worries less experience. I also got tired of updating my system every six months, so I decided to go for a rolling release.
[ul][li]Updated software[/li][li]Rolling release[/li][li]Performance[/li][/ul]
Cons:[ul][li]Some Linux experience is recommended[/li][/ul]
I'm very happy with this distro. Once you have some experience on Linux you will feel quite comfort with it. As I've said I switched to it from Ubuntu, mainly because I was looking for a binary based rolling release with updated packages. It has all the goods of Gentoo (simplicity, latest software, etc.) but without requiring you to waste time on compiling things. As you only run what you need my system boots lightning fast. Faster than Ubuntu 10.04 with all it's boot tweaking and upstart. I appreciate not having to spend an evening every six months updating my system. Another plus for Arch is that it's so close to upstream that you don't have to wait months to get the last versions of your beloved programs, some hours are enough most of the time and I haven't had any big issue with updates since I'm using it.
I've done some administration work, and from a company perspective I'll choose Debian stable. I've had good experiences with it. It's solid and supports many hardware platforms, but sometimes it's packages are too old, so you might want to use Debian testing for some non critical machines.