I use the two tools differently.
I use Seek in the field and suggest casual users use it to identify things in the field. It can work with no internet; I used it that way in Yellowstone NP this summer and was impressed. It usually gets the ID right or close. I don’t upload from Seek and mostly don’t play the challenges. I also use it to identify photos I don’t upload to iNat. I like that I can use Seek and nothing is shared about my location. I’m a user when using Seek.
I contribute to iNat from my desktop using my photos I have cropped. I understand the citizen science aspects and want to contribute because I’m outside a lot. But I’m busy, so those uploads don’t happen often. But when I use iNat, I feel like a contributor.
I don’t use either of these tools for the social aspects although my use of them involves community.