Laser Chemical Vapor Deposition (LCVD) is an additive deposition technique in which freestanding fibers are formed by the decomposition of a precursor gas under a retracting laser focal point. In this study the relationships between deposited carbon fibers using methane (CH4) and ethylene (C2H4) are compared as a function of equivalent hyperbaric pressures. The CH4 was found to require a higher laser power, or alternatively higher temperature, to initiate growth than C2H4. While both fibers could grow in a surface reaction kinetic (SRK) limited regime; the higher temperatures required in CH4 initiated gas phase nucleation (GPN) in this SRK regime. This prohibited a transition to the mass transport (MT) limited regime for CH4 fibers but is able to occur in the C2H4 fibers. The differences in dissociation and deposition are discussed in terms of molecular symmetry, bond structure, carbon availability, and the Soret effect between these two gases. The consequences of these differences in precursor growth properties then impacts the fiber morphology, including the formation of a nodular structure and graphitic crystallite size. Finally, the CH4 fibers were found to be considerably weaker in fracture strength than the C2H4 fibers, which is linked to the CH4 fibers having a higher distribution of non-homogenous flaws within the fibers.