Purchase of new mattresses

i have an antique wooden bed with two slatted frames and need two new matresses. The exact length from the head to the foot panel is 187c,5 cm and the width between both side panels is 199 cm. Each slatted frame is 178 cm long and 96 cm wide and the space between the two frames is 4 cm which explains why the old mattresses were 98 cm wide.

My questions regarding the size: the companies i turned to have all standard sizes which are obviously cheaper than custom made sizes. They offered me two mattresses with 190 by 100 each and said that the little oversize wouldn’t matter. I still could „squeeze“ both mattresses into the frame. Is this correct or should I rather go with custom made sizes? Given the provided measurements what should be the ideal size of my mattresses?

The other question is with regard to the material. I am considering either 100% natural Latex or the layer system of Vispring with cotton, wool and horse hair on pocket springs (my old mattresses had a horse hair filling). I asked the employees in two shops that are selling these products about the different qualities but received unsatisfactory answers. They basically claimed that they have similar qualities and it comes all down to the feeling of the customer. What are the criteria to help me making a decision (no big difference in price)?