A1200 going online

[pVC]

Workarounds this include configuring your LAN to work with fixed IPs (which is a pain nowadays)

It's not pain at all. Fixed IPs and DHCP can coexist just fine now and in the past. Just use fixed IPs with devices you want and DHCP on other devices if you want. If you want to make it cleaner way, define the IP-address range DHCP gives to clients from the router to something other than you're going to use statically.

or being the lucky owner of a router that supports working with DHCP while simoultaneously having some IPs reserved for specific devices (usually using MAC numbers as identifiers).

That way you're still using DHCP with all devices, but router just gives same "fixed" IP address for certain devices always (via DHCP still).

[pVC]

Why not use DHCP?

I have been using DHCP with MiamiDX from the start...

Because AmiTCP/IP based stacks don't come with DHCP support and some people prefer them for efficiency and availability.

MiamiDX is too bloated with features which are useless for many people. Makes it bit messy IMHO. I think Genesis GUI is more clear and it is already installed for people who use OS3.9. I've used MiamiDX only on special occasions like when I've absolutely needed DHCP, PPPoE, broadcasting or network sharing via Amiga, but in the most cases external routers have obsoleted those needs nowadays and basic network setups are just fine. AmiTCP is also much faster than traffic monitoring Miami...

[pVC]

As far as AmiTCP vs. MiamiDX goes, how much lighter is AmiTCP supposed to be? What's the general memory/CPU use for either?

CPU usage will be at max with nowadays speeds with 68k systems.. with faster cpu you get faster transfers and on same cpu you get faster speed with AmiTCP. I don't know memory usage as I've always had "enough" mem on my systems, but pre Genesis AmiTCP:s should take noticeably less than ones with GUIs... but mem consumption shouldn't be the issue if you have some fast mem anyway..