I went and test drove both the IS300H and IS250 the other weekend thinking that i'd end up liking the IS250 more as i'm a bit of a petrol head. However the IS300H was surprisingly good and not slow, so when you look at all the other benefits it has, very low road tax or none if you get some variants, upto 60mpg then it's hard not to choose it over the IS250. Hence why I now have a IS300h on the drive.
If you're looking at tuning the car obviously the IS250 has more potential and is easier to tune, but not many owners will be looking for that.