Lucas has a greater combo/stringing game but struggles to finish Ness off, who, whilst having more trouble in neutral against rope snake, still does good and reliable combo damage against Lucas and can actually kill. Because Lucas is letting Ness live longer, Ness has the opportunity to accumulate rage which gives him hilariously early kills against his PSI bro.
So, the way I see this MU is in Ness' favour, but possibly only by a bit. Both can gimp each other gravitational pull style using PSI Magnet, and both can somewhat circumnavigate each others' potent zoning tools (for instance, PSI Magnet against PK Freeze).