Based on a criteria of alpha keyboarded PDA
You requirement not to have a brick, but to have a decent screen size, is however contradictory, so you will ultimately have to decide the trade-off
There are five OSes to consider:
Windows Mobile (Microsoft)
Blackberry OS (RIM)
There is only one phone at present. My advice is to wait for the second generation, unless you are ok with the design.
This is a new OS being lauched by Palm. The only device announced with this, is a device called the Palm Pre. It will be available later this year.
It looks pretty nice and has good reviews, however will no doubt suffer from early adopter issues:
A great os. Lots of software for most versions and very smoothly implemented.
Reviewers complain about it being fiddly to use. It is compared to conventional phones (e..g nokia s40), however I think you need to be a moron not to get to know it. I find it easy to use.
It also tends to be a bit more sluggish then WM or BB, as Nokia is concerned about battery life, so tends to employ slower cpus then WM or BB devices.
Windows Mobile (WM)
Windows Mobile comes in two editions, standard and pro. The basic difference is that pro is for touch-screen devices and standard is for non-touch screen devices.
WM is more of a framework, as Microsoft leaves it up to individual manufactures to put their own skins on it. Therefore one can not assume that all WM devices have the same usability as it is very variable between makes and indeed models.
WM is also criticised for being fiddly to use. Again I think it is very easy to use. It is slightly more fiddly then Symbian but there nothing really in it.
WM has a huge software library and loads of freeware. Also, it is by far the most supported/hacked OS in terms of the online community. Particularly HTC devices (which are rebadged by most carriers). The advantange of this is you can get OS upgrades from hacked firmware, or firmware thats much faster then the original one, from sites such as www.xda-developers.com.
Very slick, with good integration of resident programs (e.g. windows messenger, facebook, etc.).
Limited software library and probably the poorest browser. WM browser is pretty poor as well, but most folk use opera on WM which is very good.
Also tied into carriers providing BB service. Other Oses can get email with any data connection.
Devices to look at
good retailers sites are places like www.expansys.com, www.clove.co.uk, www.handtec.co.uk, www.amazon.co.uk and www.play.com
also vodafone, tmobile, O2 and orange have good selection as well.
note that: vodafone sell all their phones without contract for a price. O2 will now sell all its phones with a 30 day simply contract.
Palm Treo 500 - WM Std - (not beautiful but cheap - only £150 at expansys)
HTC S740 - WM std - probably the king of non-touch screen WM phones at the moment
motorola q11 - WM std - new BB like
Nokia N97 - s60 - Super premium everything model. not out yet
Nokia E71 - s60 - super stylish BB competitor
Nokia E63 - s60 - cheap version of E71 without 3G
BB 9000/Bold - BB - Slick and good for email, no wifi
BB 8900 - BB - Smaller version of Bold without 3G but with wifi
Samsung i780 - WM pro - Well made, thin
HTC Touch Pro - WM pro - the king of the mini-sliding keyboard WM bunch (also called XDA Serra in O2 and Vario-something in tmobile)
HTC Tytn II - WM Pro - Brick like but well rated
HP 914 - WM Pro - Dull and business like BB copy but functional and discounts available though HP offers
Sony Ericsson XPERIA X1 - WM Pro - slick and powerful but ultra-fiddly interface
Palm Tro Pro - WM Pro - Dull and overpriced, but solid, powerful and reasonably compact
Good review site: