The bottom one would be my preferred method. Simple, minimalistic, least number of chain tensioners, and they have the option to mount the transmission on any of the three axles for weight distribution purposes.
As for chain tensioners, these can be made really simple; just use round bar stock of HDPE with an off-center hole, with a bolt going through that hole. As you rotate the HDPE bar around the hole, it increases or decreases the diameter.
Team 228 has used these as chain tensioners since 2004, and we've had great success with their simplicity and performance.
Here's a good photo I found on our team website of these chain tensioners on three different applications on our 2007 robot: our elevator (25 pitch chain), arm (35 pitch chain), and drive train (35 pitch chain in the background behind the orange chassis rail).
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