I would do that too. Dumerils can occasionally be difficult feeders until they become established. I would want to KNOW the snake will accept f/t before I bought it. My baby female didn't eat for a month after we got her. She never misses a meal now, but it was a rocky start. Thankfully, she does take f/t.
I would suggest the juvenile. I can understand being somewhat leery of a baby, especially in light of what I just mentioned, but I wouldn't want to work with a "jumpy" adult if this is your first boa.
Dumerils are a little shy in comparison to Colombian red tailed boas (BCI) and the smaller ones do like to feel more secure. However, you can fix that with the larger enclosure by filling it with a deep layer of aspen for substrate. Dumerils like to burrow so you should do this anyhow. Provide several hides and a nice water bowl, and the juvie should do alright in a 4x2.
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