The console based games are low effort (insert disk and drive), reliable (only one hardware config for the developers to test on), have more visual polish and in some cases (GT3/GT4/others?) have a huge selection of cars and modifications available. On the downside, you are stuck with the original game (typically no add-ons), the AI (computer drivers) aren't very bright and they can be overly forgiving WRT crashing, etc.
The PC based sims are can be low effort, there are typically "mods" available (usually ad-on tracks (Mosport, VIR, the 'glen, 3 rivers, Trembling, etc are available for GTL), "skins" (car graphics), occasionally cars, etc.), the AI is typically quite good (realistic and reasonably quick), the car physics are very good in the newer sims, and crashing/damage is somewhat realistic. On the downside, there can be reliability/stability issues as not all PC's are created equal (or the same), the effort increases as you add on tracks, etc., the car selection is limited and they can be unforgiving to the novice gamer.
I hadn't really touched a racing sim between getting a PS2 with GT3 in 2001 until recently with GTL and playing on-line with the MCO crowd. I'm really enjoying GTL, especially the large fields of cars in the single player mode and real human opponents when playing on-line. Kevin and I had a great race at Mosport last Friday and I had fun tonight starting from the back of a 36 car field in a '65 Alfa with the difficulty maxed out.
My Miata race car project