You know...this is just my opnion so...
Both have their strenghs and flaws... both are good for what they present and one has something the other can't. Both are preaty good, I really can't decicde becuase pratice and racing (and drifting) has been done on both as far as I can tell (maybe a bit more on Grand Valley...)... it really depends... I'm more used to Grand Valley but I can still pull off some preaty good drfits in Seattle as well...
Seattle to me is a challenging course with a lot of turns, a lot of them blind but at the same time strangley enough visible (then again this is just me, the memmory of all the track from GT-1-3 are hammered in my brain...I could attack them all in my dreams if by chance I just hapened to think about it... no Initial D refrence intended in case anyone asks...not that that would be a bad thing right?...)... you learn things on there, in a sense you would learn on ANY other track...even Test Course with super long straits that I'd figure would still not be long enough, even though it's much, much LONGER compared to the test course in GT-1/2.
Grand Valley is the same case, it has it's ups and downs, it has some preaty good turns and an overall test of car and driver ability. It's the same for Seattle and just about every other course.
Nearly all the courses in GT3 (or the entire GT serries) are good courses to drift on, each one reuqireing no less of the driver and car.
So the way I see it, I can't really decicde, I could say one of the other and later change my mind...but I've just got this feeling Grand Valley is going to be chosen a bit more than the other... but that's just me...
Till later...