I switched from a Bionic to a S4 and am very happy. I like the bigger and sharper screen, the weight, the speed at which apps run, and some of the features of Jelly Bean. Battery life for me is excellent, but I'm not what some would consider a power user, so YMMV. The Samsung apps aren't really of any interest to me, so that wasn't a plus or minus either way.
As I'm sure you know, there will always be a somewhat better phone that will come out in the future. But phones (to me) are getting to the point that I will be happy with the phone I have even if something with "better" specs comes out in the future. In the past, I wouldn't have said that. But each of us has different interests. Gets down to (1) are you unhappy or happy with the Bionic and (2) will you be happy to have a very good phone that just doesn't have the very latest in specs. And now that VZ Edge or the equivalent are available, which plan you choose.
The initial rumors regarding the Motorola X suggest that it won't be a great barn-burner spec wise.
If you're real curious about the Motorola X then I would wait a couple more weeks to see if anymore news leaks. Verizon is also holding an event next week to announce a new line of Droids so if you want to know more about that you can wait.
As for the s4, I upgraded from a Nexus and this phone outperforms it in every possible way: screen, speed, radios, battery life, etc. I couldn't be happier with the phone and after a little over 2 weeks of ownership I still have trouble putting this phone down. The features of the phone are some that I really use alot such as air view and smart scrolling, features I thought would be cumbersome and just eye candy to demo.
I really like my Nexus and it was the only phone I held onto for almost 2yrs, but this s4 is something I can see myself hanging onto for even longer and it's about as "future proof" as it gets.
I had the Droid Bionic and now the S4. I have yet to look back at the Bionic, despite loving that phone... If you're still confident in Motorola, you may want to wait, but I have to say, I'm done with that company and I'm TOTALLY happy with the S4. No phone will be perfect, however, I haven't found many faults with this device.