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Service costs actually depend more on production year and abuse the bike has taken, rather then simply which model. The later 996's are just as reliable as the early 998's for instance. The difference really comes in later in the 998 production year when they fixed the rocker arm issues. The 998FE for instance, is the most reliable of that generation.
Of course, I'd go for a 998 just because it's easier to work on.
Service costs actually depend more on production year and abuse the bike has taken, rather then simply which model. The later 996's are just as reliable as the early 998's for instance. The difference really comes in later in the 998 production year when they fixed the rocker arm issues. The 998FE for instance, is the most reliable of that generation.
Of course, I'd go for a 998 just because it's easier to work on.