Newer Miyata.
Am I right to assume the older Miyata has 27" wheels? That could limit your ability to find replacements in the event of tire damage.
The MTB is nice, too, but if you expect paved roads the whole way there is no advantage over the narrower tires on the Miyata.
Do you own all these bikes or are these ones you are considering buying? If you own them, put the low rider rack from the Grizzly onto the Miyata.
Also, for getting am ore upright position, consider swapping the handlebar stem to a shorter and/or more upright one rather than pushing the seat forward.. Quill stems are pretty much obsolete so you can get an adapter to enable the use of modern threadless stems, which are dead easy to swap out and available everywhere. A friend bought a new stem at MEC last year in Quebec City and the second half of her tour was much more comfortable because of it.
To remove the stem you will have to remove the handlebar tape... er... padding, which is good - throw that stupid black foam in the garbage and get some actual bar tape. Double wrap with handlebar tape if you want more padding.