I know Ubuntu is a fork of Debian, but I don't know how to evaluate the risks of using 10.04 Lucid Lynx built on Debian unstable. It's a home server in my basement, but clients will have access and perhaps I will host their webpages in the near future. I need to keep the server up all of the time.
Hmm, we're heading for 'subjective' territory here, but Ubuntu Lucid Lynx 10.04.1 LTS will give you the comfort of a base platform that is likely to be kept 'consistent' and 'stable' for a long period of time, compared to the frequent releases of the non-LTS versions.
You might also consider a different platform that also evolves more slowly and thus is at less risk of being tripped up by some new-fangled feature or update - CentOS would be a good example.
As to how you evaluate the different versions - without some form of objective weighting and features scoring you could only really do this by either a side-by-side trial or (next best) looking at the forums for all the distros you want to consider and reading the write ups by people who have used the distros 'in anger'.
Edit: I have just re-read the question and realise I missed out on the Debian angle: Debian has very conservative release cycles too so would also make a good choice for a stable server platform.
A good read here too: http://stackoverflow.com/questions/62222/centos-or-debian-as-a-server-os
Subjective footnote: I'd go for CentOS based on experience and the fact that it has a strong foothold among hosting providers, which is a good and relevant endorsement.
Edit2: Just thought I'd come back to mention that if you are going to offer any form of commercial (ie: paid for) hosting services, then don't forget that 'reliability' also needs you to consider hardware-oriented things such as power (UPS, redundant power supplies), equipment failures (router, server etc.) and WAN link outages. Unless, of course, you don't plan to offer your customers any form of uptime guarantee or Service Level Agreement. Also remember: in the event of natural or structural disasters, basements flood first.