MiamiDX is just another TCP/IP stack - it's not a filesharing solution itself. AmiTCP 4.x and Genesis are the same thing and are available on the OS3.9 CD. There's also Roadhshow, available from publisher APC&TCP. All three of these stacks should be functionally identical (meaning they'll all do the same thing for you). The differences lie in how you go about configuring them, and in performance.
Now, for filesharing, you'll need
smbfs to pull files from another machine onto your Amiga. If you want to pull files from your Amiga onto another machine, you'll need a
Samba server installed on your Amiga. I recommend the 2.0.7 port - I've never gotten the 2.2.5 port to work.