You may see some in the distance but as CruiserBruce says, no where near how many, and how how close, you can see them in Maui. Lahaina Roads, the water between Maui, Molokai, and Lanai, is a bottleneck for whales mating and giving birth and in seasons you can literally just sit there and watch them breach all day.
Haleakala and a whale watch is a very full day that doesn't allow for anything else on the island. I would pick one and then go to a beach or grab a nice meal and relax with the remainder of your time. Having done both numerous times the whale watch would be my recommendation. Haleakala is a lot of time crammed onto a bus when it's done in an excursion format. It's slightly more tolerable when you are driving yourself up there.