The short answer to this question is not much. The long answer (not so long plus there are some diagrams!) is below!
Q: Really not a difference between them?
A: Behentrimonium is basically a surfactant (remember this post!) with a positively charged water loving head and a very long tail. Both chloride and methosulfate forms have the exact same surfactant.
Q: So if they both have the same surfactant then why do they have different names?
A:The difference between the two is simply in the ions (negatively charged molecules) that balance the charge. Behentrimonium chloride has a chloride ion which carries one negative charge and Behentrimonium methosulfate has a sulfate ion with two negative charges. See the diagrams below!!
Q: What is 'balancing the charge'?
A: Balancing the charge means for every positive charge there must be an equal negative charge so that they cancel out.
Q: So if the methosulfate form has two negative charges, is it not better since it will have two behentrimoniums with it?
A: Not really. Technically the chloride and sulfate are present as 'accessories'. To be explicit, normally behentrimonium is effective at low concentrations - around 2 to 5%. The chemist will weigh out a quantity to make sure that the behentrimonium is present at this amount. Meaning more product if you are using the chloride form and less if using the methosulfate form.
Q: So why do I like (insert chloride or methosulfate here)?
A: You probably like the formulation of the conditioner. Okay that sounds really scientific lol. In plain terms you like the way that conditioner has been mixed. It is a bit like making a smoothie, I want extra mango in mine, someone else may want extra apple.
Depending on the condition of your hair (texture, dryness, softness etc) and the result you are looking for, you may find that one specific conditioner is really good. I have a pantene conditioner and a herbal essences conditioner, the first five ingredients are word for word, the same (same company makes them anyway!). However, the herbal essences takes it every time. My hair prefers that mix.
Does this answer your methosulfate v chloride questions? If yes, let me know, If no, let me know! Tomorrow - How to use Behentrimonium!