You are absolutely right to want to understand your options in detail before committing yourself to anything.
A lot depends on the terms of your pension scheme. Your HR department (or pensions department, if you have one) should be able to give you a statement of what you are due if you choose to retire normally (perhaps you have this already if it is something you have been thinking about?) You might already have a rough idea what you would get on ill-health retirement (based on the information you receive as a scheme member on how your scheme works, what the IHR terms are etc.) This would typically be more beneficial to you, but it depends on the detail of your scheme, how close you are to normal retirement age, and so on. If I were you, I would get the experts to do the sums for you on that as well.
A word of caution, though. IHR is not normally something the employee 'chooses' - it is something the employee has done to them (technically speaking.) So it is unlikely that you would be offered IHR as an option unless you fit the (normally very strict, on account of IHR being very expensive for the employer) eligibility criteria as laid down in your scheme rules for medical retirement. Occupational Health advice (whether in-house or provided by a 3rd party medical advisor to your employer) is usually central to this, as is HR, and your medical history, sickness record, etc.
That all sounds ghastly, I know. And it's just another bloody thing, on top of dealing with an MS diagnosis. But your instinct to proceed carefully is absolutely right, so hang on in there, and don't be bounced into anything.