Regarding size - I have used a 47cm Weber and I own several 57cm ones and also a 67cm.
Although I found the 47cm ones fine for relatively small smokes, I found that for larger things, like full racks of meaty ribs, whole briskets or whole pork shoulders it was a little cramped inside. I would certainly not be able to smoke my Christmas turkey in one - which I do each year in the 57cm.
I am not sure if you saw the difference between the Weber Original and the Premium that I brought to the meeting but there is really little operational difference between them. The main difference is the ash catcher - The Original has a tray fixed between the legs and the Premium/Mastertouch have a catching can directly under the bowl. The way the airflow is controlled by both is exactly the same.
The other difference is really only the accessories that come with it. To me, the main advantage of the Premium is that the cooking grate has hinged sides that allow you to add more charcoal/wood without having to lift off the grate. This feature I use almost every time I smoke.
It therefore comes down to price. The hinged grate can be bought separately for about £30-35 so if you see the Original on sale for £35 less than the Premium then you will be no worse off whichever way you go. By buying the Original and a separate hinged cooking grate you will obviously end up with a spare cooking grate though.
When I bought my Premium it came with the charcoal baskets but now they only appear to come with the clip-in wire charcoal retainers. Both work well though the baskets are quite convenient.
The Mastertouch does have the advantage of being supplied with the hinged grate with the central removable section. I have one of these and sometimes use it with the Weber cast iron Wok or the iron plate for frying bacon and eggs (these are sold separately though). I find I don't use them often though.
Another consideration is if you expect to be using an automatic temperature controller on it in the future. They are much easier to attach to an Original than they are to a Premium/Mastertouch.
What would I go for if I was buying one now? For general garden BBQ/smoking I would probably go for the Mastertouch as it includes the more flexible cooking grate and the charcoal baskets. It would preclude me from attaching most automatic temperature controllers though without drilling a new hole in the bottom. Is is about £40 more expensive than the Premium though so price could be an issue.
Edited by Wade - 8/13/15 at 12:41am