I have had these things for years. Having used both WM2003 and WM5, I would recommend WM2003SE. Much more stable and IMHO easier to use. WM5 seems to be more focused towards phone pda’s.
Another thing to consider is support and accessories. Not sure about Europe, but here in the US, the Acer is a relatively obscure brand for PPCs and hard to come by. Support and accessories may be hard to come by. Dell, on the other hand, is everywhere and this model is very popular, so accessories and support are much more available. This may be different there or even reversed. Just something to consider.
The RAM/ROM issue is not as important with WM5 devices as it is with WM2003. If you’re not a power user, the differences in these two won’t be noticeable.
I would suggest a Dell X50v. Physically very similar looking to the x51. It has a larger VGA screen and runs WM2003SE. This seems to me to be the best combination out there right now if you don’t need an integrated phone. VGA makes a big difference when you’re looking at video and pics. It also allows for much more information to be displayed on the screen at one time.
If the VGA display is not that big of a deal, I’d recommend the Ipaq 2755. It has much more internal memory than the other PDAs, WiFi, dual cards, bluetooth, better security features, and an integrated screen cover for a very slim and light package not needing a case to protect the screen.
Of course, these things are like cars. Everybody has a preference and someone can make an argument for any different model. Just depends on what best suits your needs.
For many more opinions from people who are really into these things check out this site.