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PLS SEE TIS : Stability of the carbocation increases with the number of alkyl groups bonded to the charge-bearing carbon. Tertiary carbocations are more stable (and form more readily) than secondary carbocations; primary carbocations are highly unstable because, while ionized higher-order carbons are stabilized by Hyperconjugation, unsubstituted (primary) carbons are not. Therefore, reactions such as the S_N1 reaction and the E1 elimination reaction normally do not occur if a primary carbocation would be formed. An exception to this occurs when there is a carbon-carbon double bond next to the ionized carbon. Such cations as allyl cation CH₂=CH-CH₂⁺ and benzyl cation C₆H₅-CH₂⁺ are more stable than most other carbocations. Molecules which can form allyl or benzyl carbocations are especially reactive.

the stability of a cation is
influenced by the electronegativity
of the atom bearing the positive charge.
The more electronegative the atom the less stable
the cation. A vinylic carbocation carries
the positive charge on an sp carbon, which is
more electronegative than an sp2
carbon of an alkyl carbocation.Therefore a primary vinylic carbocation is less stable than a primary alkyl carbocation.

also :

Carbocation Stability	CH₃⁽⁺⁾	?	RCH=CH⁽⁺⁾	<	RCH₂⁽⁺⁾	?	RCH=CR⁽⁺⁾	<	R₂CH⁽⁺⁾	?	CH₂=CH-CH₂⁽⁺⁾	<	C₆H₅CH₂⁽⁺⁾	?	R₃C⁽⁺⁾
Carbocation Stability	Methyl		1º-Vinyl		1º		2º-Vinyl		2º		1º-Allyl		1º-Benzyl		3º

SOURCE : wikipedia , web.chem.ucla.edu/~harding/tutorials/cc.pdf , http://www.cem.msu.edu/~reusch/VirtualText/special1.htm

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