I will describe the more advanced 3a here. Refer to the hardware section to see what you lose on the Series 3.
The Psion Series 3 and 3a are palmtop computers. Though packaged as personal organisers, they are fully general, programmable, powerful computers. The quality of the built-in applications, coupled with the power saving hardware make Series 3's excellent personal organisers. The sophisticated operating system, the hardware, the built-in programming language, and the options to program in C and assembler make them excellent general-purpose computers, with the major benefits of compactness and battery endurance.
Optional link facilities can connect the Series 3 to desktop machines, allowing data backup and access to the resources of the larger machine from the Series 3. Modems, fax modems, printers and other peripherals may also be connected via industry-standard serial and parallel interfaces.
The built-in applications include a database manager, a sophisticated word processor, time manager, world date/time and dialling codes database, calculator and spreadsheet. The latest models (1Mb and 2Mb RAM models) also include the spell checker/thesaurus and a patience game (solitaire card game). Many other applications are available commercially and from shareware outlets. More details in the last part of this FAQ.
The built-in OPL programming system provides a structured BASIC-like programming language with access to all the features of the machine. This includes the ability to program polished Windows/Icons/Menus interfaces like those found in the built-in applications.
The sound interface can record and playback digital sound. DTMF dialling tones can be created which allow the Series 3a to dial numbers directly through a telephone.
This question is really a personal matter. I would definitively suggest getting a Series3a (and not 3) because of the greater screen resolution. As for which memory model, this depends entirely of your needs and what you plan on doing with your Psion. In general, the more memory the better (and keep in mind also that the 1/2Mb models offer the spell checker/thesaurus and solitaire game which you might need). If you're reading this, you probably have access to Psion free/shareware also. You will see that these programs will quickly fill up your memory ;-) so I would suggest to get the biggest model (2Mb). If on the other hand, money is tight and you don't plan on using much more than the Agenda and the built-in apps, then I think a 512k is big enough for you. As you can see, there is no simple solution to this answer!
This is the latest Psion machine. Very comparable to a Psion Series 3a, it is more robust and has an A-Z keyboard. One nice point: a back lit screen is present. Targeted at the vertical market, thus not so well know to the big public.
Re-badged Series 3a, aimed at education-related markets. Contains all the 3a applications, though named differently, plus a spell checker, thesaraus and a graph plotting application in a 2MB ROM (versus 1MB for Series 3a). Password protection capability is removed. Costs about &163;20 more than a 3a.
The immediate predecessor to the Psion Series 3a is the Series 3. It is the same machine in size and concept, but is more limited in many respects. See the hardware comparison table in section 2a for a list of differences.
CM - available in 16K only, 16x2 screen, limited software
XP - available in 16K or 32K, 16x2 screen, limited software (database, OPL)
LZ - available in 32K or 64K (LZ64), 20x4 screen, introduced notepad (basic text processor), dialing codes database, on-screen clock)
See section 2.1 for the Psion Series 3 and 3a hardware specifications.
Make: Apple
Model: Newton MessagePad 120
Newton Intelligence (Handwriting Recognition, Object Oriented Database Programming language and Communications services)
Calendar (like Agenda)
NewtonMail (email client)
To-Do Lists (like Agenda)
Rolodex-like Address Book (like Data)
Digital Ink ScratchPad
Calculator (like Calc)
World Time Clock (like World)
Dictionary (13,000 words)
Notion List Manager (like Data)
Make: Casio
Model: Z-7000 (AKA: Zoomer, Tandy Z-PDA, AST GRiDPad 2390)
Date Book (like Agenda)
Address Book (like Data)
Note Book (Digital Ink Scratchpad & Document Manager with outliner)
Pocket Quicken (Financial Organiser)
America Online (Access software for the service provider of the same name)
Calculator (like Calc)
Forms Calculator
World Clock (like World)
Language Translator (26 languages & up 1000 words per language)
Games (Solitaire, Pyramid Solitaire & UKI)
File Manager
Consumer Information
U.S. Information
World Information
Make: Hewlett Packard
Model: 200LX
Pocket Quicken (Financial Organiser)
cc:Mail (E-mail client)
Data Communications (VT-100, ANSI & TTY emulation)
Lotus 1-2-3 r.2.4 (like Sheet)
Laplink (like Remote Link) for file transfers
Appointment Book (like Agenda)
Phone Book (like Data)
HP financial calculator (like Calc)
Memo editor with outliner (like Word)
Notetaker (like Notepad)
Database (like Data)
Filer (like File Manager)
Worldtime & Stopwatch (like World)
System Macros
Application Manager
Setup Utility
Make: Hewlett Packard
Model: OmniGo 100 Organizer Plus
Appointment book (like Agenda)
Phonebook (like Data)
Notepad (like Word)
Database (like Data)
Worldtime and stopwatch (like World)
Jotter (Digital Ink Scratchpad)
Geoworks Book Reader
Financial Tools
Spreadsheet (like Sheet)
Emulated HP 12C financial calculator (like Calc)
Graffiti handwriting system (handwriting recognition)
Transfer (like Remote Link)
Setup Utility (like Install)
Solitaire
Make: Motorola
Model: Envoy Communicator
Date Book (like Agenda)
World Time Clock (like World)
Address Book (like Data)
Notebook (like Agenda To-Do List)
Calculator (like Calc)
America Online (connection software for the service provider of the same name)
AT&T PersonaLink (connection software for the service provider of the same name)
SmartWallet
Make: Sharp
Model: ZR-5000 & ZR-5000FX AKA: Zaurus K-PDA
Activities (like Agenda)
Contacts (like Data) limited to 3 files
Data Files (also like Data) limited to 3 files
Notes (Digital Ink Scratchpad)
Documents (like Word) with Spell Checker
Outline (like Outline mode in Word)
Home & World Clocks (like Time & World)
Calculator (like Calc)
Filer (Manages Printing, Faxing, Email & File transfers)
Messaging (E-mail client)
FAX/Sending (FAX client)
Terminal Mode (ASCII & VT-100 emulation)
Removed for the obvious reasons ;-)