6 years would be tough. At the same time, he is the youngest of the premium free agents - the only one which you can really still project a year or two of improvement. He is a low strikeout guy - which might have more value now than it did in the more offense-heavy past, and he is a solid 3B. He is probably the safest investment of the third basemen - or at least the least likely to be a true albatross.
The Sabathia comparison is spectacularly unfair to CC, whose fat ass was one of the most reliable starters in the league for 12 seasons. It's not the favorite body type on earth, but CC got a ton of mileage out of it. It is tempting to talk about his portliness, but would be intellectually dishonest without noting that the fact he had been a 180+ inning starter from the time he was 20 has as much to do with his wearing down.
Sandoval's body seems problematic - but it has not actually hurt his production too much, improving the body is one of those low hanging fruit things that could get some bonafide improvement - which is not a common thing to get from a 6-year free agent.