There have been other reports questioning the same thing I personally think they are both pretty equal having used both. Moneywise washables were dearer for me. I have friends who came to the same conclusion.
I had twins so had to buy double I got through boxes of Ecover weekly( non environmental washing powder used in vast quantities is bad for the environment) I had to have the heating on full blast to dry the things(no tumble dryer), the liners weren't cheap or the nappy soak. I also had the washing machine on constantly yet more electricity and a massive water bill. I tooted it up and buying Nature nappies for my twins was cheaper so that is what I switched too.
I read a report that said unless you use a nappy laundering service that uses Eco washing powder washables are just as bad. The cotton used to make them isn't good for the environment either.
If I could have afforded Moltex which are compostible I would have used those and feel they would have been the best nappy to use re the environment. Financially Nature were cheaper, non environmental friendly makes would I'm sure work out even cheaper.