Ive been thinking about the same thing Mark. I have come to the conclusion that both would be desireable. I often find it a bit of a hassle changing wheels for the turbo, setting up etc, but find that you can do some good sessions on them that are worthwhile,especially in groups. The rollers however are great for smooth technique and application of power without the hassle of change over etc...so great for pre race stuff. Great also for core strength I hear. Dosnt really help you I suppose , but you need to decide if your gonna have a dedicated wheel,tire and block for the turbo ( if training on the thing ) or if the rollers without the need for specific tires etc floats your boat.
I will probably go for a turbo with a computer of some description to give read out for cadence speed, heart rate and pace. That way my training can become more tailored and more importantly keep me focused and interested.
You will probably get a better answer from a more experienced rider , but thats my take on it, hope it helps.