Okay; I've played through SAC a couple of times, and the first half the Sly 1 and the first "episode" of Sly 2. Here's what I think.
Sly:
For: The Sly games have fun gameplay, with all these "Stealth moves" which allows you to jump between pillars, or climb lamp posts or run along wires, which is really fun and really well animated, in the later games (as in NOT the first) you also get to play Bentley and Murray, each of whom have their own special abilities. Sly can pickpocket, Bentley can use poison darts to stun enemies, he also has bombs to "demolish" certain objects, and Murray can throw stuff. The characters are lovable. You have the determined thief (Sly), the clever, yet cowardly genius (Bentley), the enthusiastic "brawn" (Murray) and the slightly mad cop (Carmatella). Also, the storylines draw you in fairly well (but there's not much I can really say about the storyline, having not yet completed any of the games.
Against: There's very little to have against the Sly games, just a few minor annoyances, for instance, trying to perform a stealth move in an area in which there are multiple stealth moves that can be performed can end in Sly jumping in the wrong direction, since all the Stealth moves have the same controls (Jump and press the O button). The only other thing that annoys me is the spiders in the third episode of Sly 1, I can't stand spiders. Ever.
Secret Agent Clank:
For: The writing for SAC is a little wittier than that of the Sly games, the Sly games tried to be funny, but didn't quite manage, SAC has some jokes that, aren't great but are a little better than those in Sly. Design, Clank looks awesome in a tux.
Against: LOADING TIMES! The loading times of SAC are appalling; it can take almost a minute to load each section, whereas Sly loading times were very, very quick. Gameplay, Stealth doesn't really work when you can't pull off any sneaky moves like Sly can, the stealth in SAC just involves sloppily tring to run between spotlights and occasionally blocking laser emitters with ink.
Overall, I'd say Sly is a better series than SAC is a game.