Is a Chapter 5-17 a guaranteed thing?

MEBs are initiated by your PCM, 5-17 is an administrative separation. In the first one you are compensated for your injuries, in the second one you are kicked out with nothing. If you're a PFC you won't even earn your GI Bill.

If you are legitimately injured then do not ask your command to chapter you. I can't begin to tell you just how epically stupid that would be and how many hundreds of thousands of dollars you would cost yourself over the rest of your lifetime

Your PCP is the one that recommends/initiates the MEB process. If your profile has a P3 or P4 in any of the PULHES, then a MEB should be auto initiated. The issue, that I'm guessing, is that they want to go the Chapter route because it would be faster than a MEB. Is the unit provided physician your dedicated PCP or do you have another one?

PFC (Join to see)

If you have physical injuries back, shoulders, ankles etc I don’t believe you will receive a 5-17, so in essence it is not a guarantee. In most cases the MEB is the first step of the medical discharge process. A service member with a medical condition that makes them unfit for duty is referred to the Medical Evaluation Board. The Board reviews their conditions and makes recommendations to the Physical Evaluation Board on which qualify as unfitting.

Definition of a Chapter 5-17
Chapter 5 of the Army Regulations, called Separation for the Convenience of the Government, sets out many administrative discharges, including Chapter 5-17: Other Designated Physical or Mental Conditions. This is comparable to Chapter 5-13: Personality Disorder, but Chapter 5-17 covers conditions that don't meet the medical criteria for "disorder." Examples of conditions that fall under Chapter 5-17 include: chronic airsickness or seasickness, dyslexia, sleepwalking and claustrophobia. The key factor is that the condition causes disturbances, thinking or behavior severe enough to diminish a soldier's ability to carry out his military duties and tasks. Before triggering the Chapter 5-17 separation, the commander must make sure the soldier in question has been sufficiently counseled and offered a chance to be rehabilitated. In some cases, rehabilitation involves changing to a new section or platoon within the same unit or to a new unit entirely. The soldier is advised to consult with an attorney at the beginning of the process and if he opposes the Chapter 5-17 separation, he can put forward a case for consideration before a final decision is made. However, only soldiers with more than six years of active and/or reserve service are entitled to challenge the decision at an Administrative Separation Board.

Soldiers discharged on the grounds of Chapter 5-17 receive an honorable discharge unless there are specific elements of their service record that warrant a general discharge. Soldiers with up to 180 days of active duty service may get an uncharacterized description of service. Soldiers with at least six years of active duty service who have completed their initial period of enlistment may be entitled to separation pay following a Chapter 5-17 separation.