re: Which is better: hex bar or straightbar deadlifts?

You should be able to hit 135 on hex bar pretty easily even if you're just beginning. Hex bar gives you a good body position.

If you're concerned about back, more than weight, bracing will help in safety.

Here's a good video on bracing and stabilization while lifting. Nailing bracing will be great for you in all major lifts.

LINK