There are two different types of date book replacements. One uses a proprietary database format and completely replaces both built-in PIM applications and conduits. Beyond Contacts and KeySuite by Chapura are the two applications that fall into this class.
The other type is a program that uses the standard Palm PIM databases and conduits but essentially rides on top of them, adding things like linking and icons. Agendus and DateBk5 are the two leading programs in this class.
In terms of RAM usage, the total PIM replacements like Beyond Contacts use the most memory. IIRC, Beyond Contacts is about 1.3 mb and KeySuite is about 1.4 mb. The enhanced PIM applications like Agendua and DateBk5 use far less memory, even with icon collections. Both are around 800k. Both applications use tagged note fields for things like icons, colors, and linking because the standard Palm databases don't have fields that handle these types of options. Both programs will also allow you to link appointments with contacts and to-do's but DateBk5 allows linking to memos as well as allowing multiple links in both directions. Agendus has an option to add pictures to their addressbook, something that DateBk5 doesn't have. The cost is memory, with each picture running about 50k.
Agendus is notorious for frequent updates that you have to pay for. Some users have estimated they would have spent about $110 if they had gotten every update since the program's inception. DateBk5 has had only one major update that you'd have to pay for and even that you could get for free if you were able to use the automated update system.
Beyond Contacts and KeySuite are ideal if your main goal is to have as close a mirror to Outlook on your Palm as possible. They have more contacts fields, the task fields match Outlooks task fields, and they allow things like multiple categories and transferring Outlook links, something neither Agendus or DateBk5 can do.
I'm a confirmed DateBk5 user so my opinion is biased. I've tried both Beyond Contacts and KeySuite and neither had features like icons, templates, and saved views that I've come to depend on in DateBk5. If I was going to switch, however, it would be to KeySuite. It's more expensive than Beyond Contacts but Chapura has extensive Outlook experience through having Pocket Mirror as the standard Outlook conduit included with Palm's for many years. They also have an unrivaled knowledge of the Palm OS and I found their customer support to be very good. Beyond Contacts was not as stable as KeySuite and it seemed a lot slower syncing than KeySuite. Both have free trials so you should try each one and see which might be the best for you.