Here are my thoughts. I own two current models and they happen to be an hp4150 and a Tungsten T3 (and a Palm IIIc

) So I should hopefully be able to give some kind of unbiased opinion.
First of all, IMHO, there is no clear cut winner. I used to be Palm all the way, but since owning an hp4150 I have discovered certain PPC benefits, so now I find it hard to recommend outright one way or another.
1) Screen. 4150 vs T3= T3 wins hands down. 4150's screen is fine, until you compare it to the T3's. T3 has more pixels and MUCH better colour (although I dare say the 4150s screen is better in the sun.)
The extra pixels are awesome on the T3 when it comes to pictures, but not quite so relevant with text, as 4150 has 'clear type' which hugely enhances text appearance. Only real benefit I can think of with the 4150's screen is when watching movies. On a 4150, you can run a full screen movie easily. The T3 has so many pixels, it's hard to encode a full screen movie that will play great on it. Then you're down to using zoom which ruins quality.
2) Battery. 4150 wins hands down. It lasts nearly twice the time of my T3 on average use (sorry Palm guys, it's true). This is party down to the T3 shutting of at around 20% power, the 4150 continues till flat (then a backup battery preserves data for a short while). I'd say 3.5 hours on average Palm use and around 6 hours PPC use, both on lowest brightness and no wireless on.
3) Wireless. I have found WiFi useful since I had it, although I didn't think I would before I had it. It does drain more power, but with WiFi on (and in powersave mode) I'd compare battery life to that of the T3 (with bluetooth off on T3). I can't say I notice any effect on battery life while using bluetooth on the 4150 (though I rarely use it).
4) Software. There is lots of good freeware for Palm and Pocket PC. It's fairly straightforward to port PC programs to PPC (I have heard) and therefore there is lots of good pocket pc freeware out there. The best audio player and the best movie player (in my opinion) for PPC are both free, which is a plus. As are fully customisable book readers, picture viewers, zip/unzip programs, emulators etc. You will find these too on Palm, but I'm afraid to say I think the array of free software is not as good as pocket pc.
But, as has already been said, PPC software is much much bigger, so you'll fit a lot less programs on. (Using a large SD card solves this though.)
5) Speed is a plus for the Palm, in general use. Although when it comes to movie playing and background music, the speed seems to be used more effectively in the PPC world, although I'm sure this is down to how the OS and Program software is written.
6) Documents. I'm happy to say here, Palm wins hands down.!Documents to go is WAAAAY better than the Pocket Word/excel on the PPC. I think this is a big Palm plus. As is PIM syncro (much better on Palm).
7) Size/Quality. The 4150 is slightly smaller and lighter, although it's much bigger in it's case, and the T3 has a nice flip cover.
Despite the T3 being bigger, it feels nicer and more sturdy. The T3 body is made from some kind of metal alloy (it is NOT made of Tungsten!) as opposed to the plastic body on the 4150. Furthermore, the T3's stylus is vastly better to the 4150's, although hp kindly supply you with two in the box which is useful!
I have to mention my issue with Palm digitisers too. All three Palm's I have owned have had this issue with 'digitiser drift' which only seems to worsen over time. Partly for this reason, I currently use my 4150 more than the T3. I don't think I've ever had to redigitise my 4150 since the day it arrived.
iiicRuled