So you are trying to choose between a Lego Chima and a Lego Hero Factory set? I don’t think there is any issue. Both are great. I mean, look at the amazon links above to see the endless pages of 5-star reviews parents have given the sets. Kids love them both. My boys (6 and 9) play with their Hero Factory and Bionicle sets for HOURS and HOURS.
The only thing with as much or more staying power in this house has been the 2-sets of 100-piece magnatiles we have.