Hi Jake
Visit http://www.ukrcc.org/
This is where the frequency allocation comes from. 27MHz was historically the model control band some years ago an american craze brought illegal CB into the UK and screwed things up especially for aircraft modelers ( if you use a transmitter on the same frequency as someone else nearby they interfere and in the case of controle systerm you can loose controle)
As shown on the above site 35 and 40 MHz bands were allocated to allow modellers more freedom to practice their hobby.
As far as equipment is concerned, 27MHz is usefull if you want to fly or sail or drive models and use the same equipment. 40MHz has more channels available but NOT for flying.
You don’t say why you want this answer, if it is to decide what equipment to by then it depends how much you can afford I have 27MHz gear but I am phasing it out and replacing with 40MHz altho some ‘serious’ modellers are moving to the more recent frequency allocation 2.5GHz. The literal answer to your question.
The difference between 40MHxz and 27MHz is 13MHZ.
Hope something in this helps
Cheers