Fotu is likely going to be taken in round 1 or two, and will likely be Utah's highest drafted player since Star Lotuleilei. I could see Tonga starting over Penisini potentially... and he would at the very least get lots of snaps. I will give you that.
Utah doesn't run a 4-3 scheme anymore (not most games anyways). They spend most of their time with 5 defensive backs and two linebackers.
Tonga and Penisini have pretty much identical stats, with Penisini having a handful more total tackles... same number of sacks. Tonga COULD start over Penisini, but it's hard to say because there are a lot of variables when comparing these types of things.