The difference in feel between the 135cm and 150cm should be negligible.
Are both mattresses on the same type of base?
Have they both had equal use with similar size people using them?
If you are considering buying a new 150cm mattress then our recommendation is that you try our new range of products now available in store. The chances are the fillings in both your existing mattresses will have settled somewhat over the three years so any new mattress is going to feel a little strange at first and you should bear this in mind when trying them. Narrower your choice down to one or two and then lie on them for at least 10mins in your normal sleeping position to guage which is most supportive and gives you the best level of comfort.