Which has a better opportunity to promote past E5, 11b or 12b?

12B by all means, all the fun and excitement of 11B plus demo. The Combat Engineer units have slots coded for both Ranger and Sapper, as well as the Airborne and Air Assault units, so again the best of both worlds. But I'm biased as I spent my career as an Army Combat Heavy Construction guy supporting Infantry, Cav and Artillery units.

11B hands down the fastest promoting MOS across all the ranks. It's the largest MOS in the Army. Even my Special Forces CSM was saying how his former 11B peers beat him to E9 years ahead of him. The promotion points are usually low in the junior NCO ranks and the selection rates are consistent in the senior NCO ranks. You'll still have to distinguish yourself from your, and Airborne, Jumpmaster and Ranger school are a good start.
In fact, whatever MOS you pick, those three schools should be your immediate goals. Ask for airborne school in your contract.

Dude, I’d go 12B and go to Ranger school, then after 3 years or so, then put in a 4187 and switch over to 11B. Once you’re Ranger qualified, 11B becomes your secondary MOS. And a 4187 gives you the opportunity to switch over. Get that Sapper tab as a 12B too.