Hello Jjbeau,
First if you search this King and Country forum folder I am sure you will find this topic in dozens of different threads.
*** THERE is NO true answer to this question ***
I would hazard a guess that most collectors have toy soldiers from many different vendors. I have at least three manufacturers that I focus on [K&C, John Jenkins and Frontline] , but K&C is my first choice.
I am sure everyone will have a totally different opinion of why they choose to buy from one company over another ... AND everyone is correct. It is "not correct" to say that any one manufacturer is better than any other. It is just a matter of taste and the ranges that they make; the price that they charge; the scale that you like .... and so much more.
Another word that I will point out is your choice of the word INVEST. As much as I love my hobby, I would hesitate to use the word Invest. Yes, many items, from all manufactures do increase in value, but some do not. You should purchase the items because they are fun and will make your life a little happier but NOT as a investment.
So my take is ... buy from whatever manufacturer you want, get the soldiers that you are interested in and have FUN, FUN and more FUN
---Larry