I bought a 4150 last week, after having a Dell Axim X3i on trial for a week -- and reading several reviews I found on the Internet. You can search using Goodle, check out this forum , and also
http://www.aximsite.com
I enjoyed trying the X3i - but it doesn't support Bluetooth (from approx May 2004) - which was very important for me. I think the X3i had more memory than the 4150, and was quite a lot cheaper too.
However, the 4150 comes with more standard apps - but I couldn't tell you what they are... maybe HP Printing was one of them...!
I'm sure you will be happy with either unit to be honest.