GT5 has more tracks, and more variety of tracks as well (off-road courses, go kart tracks etc) combined with the ability to create your own and race them online as well (so in essence, it has an infinite amount of tracks). As for quality however? It varies. Some tracks in GT5 look utterly spectacular (city courses in particular) but others look awful. Thankfully the amount of ugly tracks isn't too high, but it's still a shame to see.
As for Forza 4 however, all of the tracks are of a high quality. Well, one exception would be Silverstone in my opinion which looks hideous (and Fujumi Kaido as well now I think about it), but it's not like Turn 10 had much to work with anyways given how it looks in reality, so that's excusable.
One thing that I find frustrating with Forza however, is that many of the tracks are in my opinion just not at all enjoyable to race around. This is all personal taste however, and something that you'd have to decide for yourself. I find GT5 to have better tracks in the sense that they're more fun to race around, but it is worth noting that Forza does have some excellent courses such as Bernese Alps, Maple Valley and Camino Viejo.
If you're really keen to compare tracks, you can find a list of the tracks in Forza 4 on this forum, and a list of the tracks in GT5 over at gtplanet. There are plenty of videos on YouTube which shows the various courses in both games as well