There is a significant difference between the two. The former, primarily JVP, enjoys a fairly strong ability in mobilizing masses, mostly youth, in comparison to the latter. Though the voter base in comparison to the two major parties (UNP and SLFP) which have been in power in Sri Lanka since independence has been significantly low, JVP is indeed a force to be reckoned with. This is, to a larger extent, due to the two armed insurrections staged by the party since independence. So the JVP's mobilizing power is unquestionable and has a fairly strong presence among the students of the country's state universities. The Samasamajists and Communists voter base is, however, significantly low in comparison to JVP and has been able to represent legislature while being party to the ruling parties especially with Left lenient SLFP. So the Samasamajists and Communists enjoy a significantly lesser mobilizing ability of masses. There is a bit of a bizarre scenario here. The Marxist revolutionaries are generally to do more with their ability in mass mobilization. In that sense our old revolutionaries are under-performers to a large extent.
In order to become the followers of an ideology one needs to have a clarity about that ideology. This is where we find a significant divergence between the two Marxist types in Sri Lanka. While the well- read old Marxists have been fairly clear about the Marxist ideology, the JVP types have been a confused bunch with shabby and incoherent knowledge on the ideology which they follow dearly. The founder of the JVP himself was suspected to have been incoherent and confused about the Marxist ideology. This major gap on the part of no lesser quarter than the party leadership itself led to some of the intellectually sound cadres to leave the party in 1970's and early 80's. In other words we find a large body of masses, primarily youth, who strongly follow an ideology with an incoherent knowledge about it. They make up, however, the largest body of the masses who represent the Marxist Ideology in the country which even staged two uprisings resulting in massive destruction to the lives and property of the country.
The very fact that the present JVP leadership is supporting the blue- blooded Capitalist party which governs the country now is a clear example of this. On top of all it is clear even to a political nit wit that the present ruling party represents and follows the agendas of the Western Imperialists. A central axis of Marxism is its rejection of capitalism and the Western imperialism which the ideology believes as major exploitative forces of masses. In other words the JVP fundamentally goes against its Marxist ideology, However, it is still the largest, vibrant and active Marxist party in the country. In doing so the party furthers and strengthens the Capitalism and the Western Imperialism which literally sells Marxism. JVP is also widely accused of receiving a substantial amount of funds from the US during the presidential election in which Sarath Fonseka contested. The amount is said to have gone into thousands of dollars. Wikileaks have revealed that JVP has misused the US funds. Needless to say that the US has been the citadel of the Western Imperialism and had been playing the biggest role in almost wiping out the communism from the face of earth.
In contrast the Samasamajists and Communists have, to a larger extend, been following nationalistic policy maintaining a steadfast anti-Capitalistic and Imperialistic stand. While being part of the SLFP led governments they have been able to stand firmly to preserve the country's sovereignty steadfastly opposing the Western Imperialists' interventions to the country.
In an overall assessment there are two scenarios. On the one hand there is the JVP, the largest and the most vibrant Marxist party in the country which seems to have not understood the fundamental ideological tenets of Marxism. This has led the party to be a promoter of Capitalism and the Western Imperialism in the country, a worst ever possible state of affairs expected from a Marxist party. On the other hand we got Samasamajists and Communists who have never lost the ideological core of the Marxism in principal and actions. Besides, they have contributed to the country whenever and whatever possible ways while aligning with Leftist oriented ruling coalitions. However, they woefully lack the mass base which their more radical brethren abundantly enjoys.