In-Cabinet (sink base) electrical box - Metal or Plastic?
I am assuming you are trying to install a switch leg for the disposal and a continuous hot for the dishwasher and the disposal and, dishwasher are on the same circuit, and you are trying to use a split duplex for both devices. I am also assuming that you are trying to surface mount the box inside the cabinet rather than use a cut-in box. 

If you are installing a surface mounted box the conductors or cabling would have to protected no mater what kind of box you are using. If you came out of the side you would have to protect the conductors until you could pass into the interior of the wall or some other protected area. 

One other problem exists the dishwasher circuit requires GFCI protection and the disposal doesn't. So if you do not have GFCI protection for this circuit you cannot put a split receptacle in under the cabinet. You would have to install two separate receptacles (at least one GFCI protected) or add a GFCI breaker to that circuit. 

Taking this one step further. Since the NEC requires at least the dishwasher to be GFCI protected, it would be safe to assume that the area below the sink is considered by AHJ to be a wet location which would mean an "outdoor gang box" would be required in a surface mounted box.

FYI - The new 2020 NEC has modified 210.18 (A) which will require every 120V outlet to be GFCI protected including disposals, as well as any other wet location inside and outside a dwelling.

Hope this helps