I don't know about the resale value of Unicomp keyboards, but I am assuming it would be lower than original purchase price. At least if you pick up a Model M within a decent price range, you are pretty much guaranteed to be able to sell it for what you paid. I guess from that perspective getting a model M over Unicomp even if it to just try it out would seem safer. If you find it in a garage sale and you didn't like it then there is a chance of making a small profit on it.
As long as you don't do two days research, decide on something a bit rarer like an SSK, see an opportunity, get a case of the crazy spenders and pay a keyboards worth of shipping (not to mention the sale price) for your first Model M. Straya problems.