Two years later (since my previous post here) I've got to use Ubuntu on a double Turion HP, have installed Puppy 431 on a USB key just in case I'll need it, and will soon install the same Puppy on my old IBM PII-266MHz & 64MB Ram (tried it Live-CD and works). From the Puppies, I liked most the Wolf's desktop, but didn't work with the PII.
It took me so long, because there are so many releases and absolutely no help - everybody say the same thing: try as many you can and chose by yourself the flavor you like - Thanks, but...! This is why I would like to share here the key issues helping me to make a choice:
1) I think the most important is to decide what desktop you like: mainly Gnome or KDE, but also the other more on the light side desktops. In this respect downloading a big release (only one, no matter which) with all its desktops seems the first step to make. Try the Live-CDs and decide what are you fine with.
2) After choosing the preferred desktop, it's time to decide between a rolling release and a long term one (LT). The LT should be a better choice because, if you take 2-3 weeks of vacation and do not update your Linux in between, when you return the update and possibly the upgrade are smooth (at least with Ubuntu was so). In the same conditions, but with a rolling release, you might be roasted - using Sabayon, the most beautiful Linux I've ever seen, but a rolling release, after 3 weeks of not updating/upgrading it, when I finally did it, I've lost all my desktop (task bars, icons, settings, all). Fortunately I could backup my documents using the right button of the mouse on the empty desktop - the only way to open a location on the broken system and to copy it on a USB key. This is also why for my old PII and my USB key, I choose Puppy over AntiX. For me LT is a must, not a choice.
3) With these, the choices narrow down to very few releases (2-3). Try them on Live-CD, possibly install them next to Windows (double boot), and if you don't like one, install another over it - very easy to install on the same partition created by the old one. As for the choice, I would say that the main criterion is the availability of programs you need. Some know exactly what they need, some other just need time to discover the new possibilities Linux offers them, and get used with the very wide range of free (of money and spam) applications, compared to the very narrow one Windows offers with lot of spam and for lot of $$$. Hobbyists will find their Paradise. Moreover, installing and uninstalling apps in Linux do not leave traces like in Windows, so trial is almost unlimited.
All these, most of you know already, but new Linux users don't, like I didn't 2 years ago. And I hope this will help them to save lot of time when choosing the release the most appropriate for them. To me it took a year and a half - much to much, and in between I completely neglected my darkroom! So, if I wouldn't have hate Windows so much, I would have given away the Linux trial after only 2 weeks, so difficult was to chose over such a large diversity and without any criterion or landmark. So, I hope what I wrote here will be useful for some newcomers to Linux.
Best regards!