I replaced my Bionic that I'd had since the day after it was released with the S-4 on June 15. I've had the S4 long enough to live with, which has included some traveling. I feel that at this point I can offer a realistic assessment of the S4 compared to the Bionic.
In a word, the S4 is simply a much better device. I had rooted my Bionic and was using an extended battery. The S4 is faster, has a better screen and user interface, and gets substantially more time out of a standard battery than the Bionic did from a moto extended battery. The only area where the S4 it might lag behind the Bionic is in durability. I have installed a body glove case to protect against damage from the eventual drop.
It seems that your choice is more one of a carrier to get the best bargain. Once you narrow that down, I think you'll be very happy with the move from Bionic to S4.