If you are working in a known advertising agency then I think you should stick there are learn the tricks of the trade. Having an advertising background myself, I can vouch that you will get tremendous opportunities to learn and grow at the lower levels of an advertising agency, and these will give you a solid foundation to build your career upon.
Client servicing personnel are easliy able to transition to the client side. So it will be for you to see whether you require an MBA afterall or can effect that switch without the MBA degree. Needless to say, you will get a much higher position post MBA, making it worth the time and effort.
All the best,