In situations like this if the client is being difficult it as about 2 things - cost or flexibility.
Costs - if they are pushing the CMS choice purely because of cost - then i am happy to walk away from such clients (a nightmare tends to follow).
Flexibility - If they are making choices based on a product they like, their employees are used to (more efficient on), believe they can jump developers more easily, and they are making an educated decision - then heed their advice/opinion.
Long and short of this is - if you are not the right person for the job as the CMS does not suit you, or you are not efficient on that CMS - then advise the client on this honestly. I would rather let a client go based on good trustworthy advice at the beginning, than lose a client halfway, or at the end based on not being right for the job.
This approach tends to get their trust and in many cases where i have let them go early on - i find them returning after they were burnt elsewhere. This is especially the case based on price fixated clients - who get burnt on stupidly low quotes.