Its really down to what you would be better having. A one hour lesson can be a bit over whelming to start with, it feels like you are having a lot thrown at you at once, but having a one hour lesson also means you lose time at the beginning and end recapping so may not spend a huge amount of time driving. Overall 2 hours may be slightly quicker as you don't lose as much time per lesson, but if you find that after an hour or so you have had enough and start making silly mistakes etc (I used to struggle to concentrate for more than an hour) then ultimately you are better only having an hour than 'wasting' the 2nd hour by not driving well.
The other thing depends on where you live, I lived pretty near the test centre to about 5 mins and I could be on a test route. If you live further away you may struggle to practice test routes in a one hour lesson.
Prehaps start having a one or two hour lesson, whichever you prefer, and see how it goes. If you have one hour and feel you could have more then try a two hour next time...if you don't like two hours just have one. It is totally up to you. Which is another point...obviously listen to your instructors advice, but don't feel that you have to go by what they are saying. At the end of the day you are the 'customer' so decide what you want. Sorry - have just heard various storys of instructors telling pupils they *have* to have two hours etc. Which may not be right for that person (it wasn't for me) but it means that the instructor manages to get more money as they get 2 hours of fees, but without having to pick up a 2nd learner - if that makes sense.