I am heavily biased, as I hate superheavy protos exactly because they clash with ultralight mechs, and there is no speed scaling balance so the 15 ton protos just are superior to 9 ton protos.
The superheavy proto here is better, as among other issues the proto doesnt have a pilot skill, so cant fall and cant skid, and losing legs isnt really even a bother. Also, because of how stacked superheavy protoarmor is, needing 3 direct 15 point hits to the torso to kill the superheavy proto is a lot of damage.
The ultralight, by contrast, is dead to 15 damage to the center, and has a rear armor to worry about. Also, losing a leg hampers the ultralight, who can fall, a lot more and messes with its shooting having to prop and fire. The BV is less, but only by a slight amount, while the proto needs 3x the damage to kill or cripple most of the time, as even the legs can take a direct gauss shot with no issue. Heck, the superheavy proto can survive a blazer to the face! Like, what!?
Anyway, my bias against the overtuned superheavy protos got the better of me, again. I have no doubt a 20 ton 'super-duper' protomech would be better then any 20 ton battlemech too!