Agree with the above; some HA D3 coaches do have connections with summer leagues, and, more to the point, encourage their players to play in them. Some players at HA schools are mostly focused on their academics, and some are more focused on baseball. It's possible, now, to transfer from a D3 to a D1 without sitting out. So I would argue that going to a D3 is the better choice in some ways, because that way if you don't move "up" in baseball, you're still at a great school.
The main argument for juco instead of HA D3 would be for the baseball training. The facilities, coaching, etc. are not the same at a HA D3s as at many D1s or jucos. So the possibilities for development are not the same, and more of it is on the player, which is, frankly, hard. What I would want to know about a juco is, what happens to all their players who don't go on to power D1 programs? What would be your choices then, with or without baseball?