Luigi's Circuit is awful. Mario Kart
Wii's biggest problem is that there are just too many racers. All Mario Kart games have crazy items that take little skill to use, but when you have twice as many people using them on a course that's pretty straightforward and requires little skill, it becomes ridiculous. Luigi's Circuit basically always boils down to who gets what items and when. It is easy to get hit by a star, bullet bill and more all at once simply due to how many people there are and how the course is layed out. If there were only 8 or so racers, Luigi's Circuit wouldn't be too bad. 6 would be even more reasonable.
Mario Kart
DS has worse wifi, but the wifi is much less frustrating simply because you're only going up against, at max, 3 other people.