This is just a minor conflict, but both of these mods are very high quality and this is the only way in which they clash.

Preferred resolution

• Terrestria uses Biome Makeover's Saguaro texture, model, and icon. The needles stick off of the body a little, matching vanilla cacti. And the model never creates loops when you place several blocks together.
• Terrestria adds a composting recipe for the Saguaro to match vanilla cacti.
• Biome Makeover uses the Terrestria Saguaro sapling so that their cactus can be grown.
• Biome Makeover's cactus uses the terrestria texture to show that the cactus has been damaged.
• Users may choose to hide one or the other in REI to avoid duplicates showing even though both are identical.

The same issue: Lemonszz/Biome-Makeover#11

Obviously renaming one or the other is another easy solution =]

Oh, the Biome Makeover saguaro actually grows from individually placed cactus blocks rather than from a sapling. That might be a better behaviour for matching vanilla.

I resolved this for myself by renaming one "Sonoran Cactus."