CAESAR is the correct spelling. "Ceasar" is incorrect. Remember that in Classical Latin it's pronounced similarly to "Kaiser" (whereas "Seezer" came later from Vulgar Latin).
http://en.wikipedia.org/wiki/Caesar_%28title%29
http://dictionary.reference.com/browse/caesar?s=t
Interestingly enough, Takara used both Classical and Vulgar pronunciations in 1989/Transformers Victory. The Decepticon Breast Force gestalt Lio Caesar (ライオカイザー) is pronounced "LYE-OH KAI-SAR" (hence it's often Romanised as Lio Kaiser by Western fans). The Autobot Brainmaster gestalt is called Road Caesar (ロードシーザー) and uses the Vulgar Latin pronunciation of "ROAD SEE-ZAR" (and thus English speaking fans still spell it as "Road Caesar" since the English pronunciation of Caesar is based off Vulgar Latin anyway).
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