"I'm in the dark about chisel mortisers. Are they available in the USA? Opinions of chisel vs. chain?"

Makita and Ryobi used to make and market chisel mortisers in the US. But I haven't seen them for years. You might find a used one on Ebay or through advertising in Scantlings.

As to any opinion on their use, that depends on your use for them. A chisel mortiser is, in my experience, much slower at producing a standard mortise(say 6" by 1/12"). However, when you're done, there should be absolutely no chisel work needed. And as my earlier response comparing the flexibility of the Makita chain to the Mafell chain mortiser goes, you might find the chisel mortiser more flexible yet.

If speed is of absolutely no concern, and you can wait until you locate one, then try a chisel mortiser. I would think that like the other mortisers, there is a strong enough market that you could recoup most of your investment should you decide to sell it later.

Personally, I would not use a chisel mortiser to produce a timber frame; it just seemed too slow. But what you could do is buy a Makita chain mortiser (new or used), keep looking for a chisel mortiser, use them both for a while, and sell the one you like least.

On a side note, working recently in England, I saw many "mortices" (that's English for mortise) very quickly drilled and chopped with a corner chisel. It demonstrated how tooling is primarily about comfort levels and practice. This, of course, would be the cheapest approach.

On a side note to the side note, utilizing this method also seemed to requir smoking hand-rolled cigarettes.

Good Luck,
Bob