Which one should i get?
They're both very good, but like the above poster I preferred Sly too, but you can't go wrong with either.
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I've personally picked up The Sly Collection last year as the original Sly was the only one in the series I played back when the PlayStation 2 was still in full swing. The Jak series on the other hand is one I've already played through on the PlayStation 2, which in turn are all great games. Preferentially, I'm a Jak fan much more than a Sly guy.
The Jak and Daxter games are awesome. They're on a level story, graphics, sound, and gameplay-wise that the Sly series just doesn't hold a candle to. Not that the Sly games are bad; I'd call them average at best. Jak's series though is one of the greatest achievements the PlayStation 2 has seen. If you're looking for a stronger game then obviously Jak is your ticket to greatness.