I liked Bonnie better she had a better relationship with Michael and KITT, I didn't really see that with April, she seem to be more of a mother to KITT and Michael. Bonnie seem to know more computer stuff.
I liked Holden's acting; I thought she played April very well. I also think that since McPherson had the chance to start with the rest of the series, she made more of an "imprint". So, I also have to cast my vote with her. :)
Bonnie or April....I would have to say Bonnie...She was the better actor and seemed to be smarter and love hate relationship with michael...April is better looking...but intelligence is sexy so I give the edge to Bonnie....I had a girlfriend who felt threatened by my crush on Bonnie