I am a little biased so take this with a grain of salt.
mTCP was designed for an 8088 class CPU. As a result, it is blistering fast on later hardware, like 386s, 486s and Pentium class machines. On a faster machine (Pentium 133 with 100MB/sec Ethernet) you will be limited by the speed of DOS accessing the hard drive, and not the network (Running Windows with real 32 bit hard drive access and DMA should be better if you are going to transfer tons of data.)
I would much rather run a batch file to load a packet driver and get a DHCP address than I would start an entire Windows operating system just to transfer some files around. On faster machines starting Windows is not a big deal, but on a slower machine that is painful. Most packet drivers can be unloaded from memory easily which saves time when you want that memory back.
The DOS MS LAN client can do file sharing with drive letters which is a nice feature to have on occasion. But it is a real memory hog. I have it installed and use it on occasion, but it is so rare that I often forget how to get it going again.
With the mTCP FTP server running on the DOS machine you should be able to connect using the Windows Explorer. I'm pretty sure the Windows Explorer understands FTP. If something is glitchy let me know and I'll try to fix it.
And remember FTP works whether the server is in your house or on the other side of the world ...
Lastly, mTCP is proudly developed using ... DOSBox! (Most of my testing is done using the HAL9000 build.) It is not hard to install. Just read the docs .. I spent a lot of time writing good docs, so use them!