Ive got both.... and ive completed saints row.... took me a total of 24 hrs (a bit of a let down but was still a good game) . 'Just Cause' Is absolutly massive if you have ever played Oblivion IV: Elder Scrolls ... the map is massive .... well Just Cause is even bigger. if offer the same principals as GTA: San Andreas jus better in my opinion! its Soooo Much fun to play. you wont move away from your Tv!
jus look at the size differences..
Oblivion = ~16 sq. miles
San Andreas = ~17 sq. miles
San Esperito (Just Cause) = ~390 sq. miles
So its not as if your goin to get bored of exploring.
Hope it helps