Parkinson's disease (PD) and the parkinsonian variant of Multiple System Atrophy (MSA-P) are two neurodegenerative diseases which share similar clinical features, particularly in early disease stages. The differential diagnosis can be thus very challenging. Dysarthria is known to be a frequent and early clinical feature of PD and MSA. It can be thus used as a vehicle to provide a vocal biomarker which could help in the differential diagnosis. In particular, distortion of consonants is known to be a frequent impairment in these diseases. The aim of this study is to investigate distinctive patterns in the distortion of voiced obstruents (plosives and fricatives). It is the first study which attempts to examine such distortions in the French language for the purpose of the differential diagnosis between PD and MSA-P (and among the very few studies if we consider all languages). We carry out a perceptual and objective analysis of voiced obstruents extracted from isolated pseudo-words initials. We first show that devoicing is a significant impairment which predominates in MSA-P. We then show that voice onset time (VOT) of voiced plosives (prevoicing duration) can be a complementary feature to improve the accuracy in discrimination between PD and MSA-P.