I have a 24x50 Shirley and I have found it is plenty big for my home use. I went with a flat-back raised cabinet model.
Things I would recommend/do different:
1) The shelf on the side of the warmer is really handy. I find myself using it a lot.
2) I would possibly nix the warmer, but if you do want it, get the optional block that lets you cut off airflow to the main chamber to make the warmer function like a vertical smoker. The block wasn't available when I got my smoker, so I tend not to cook in the vertical for smaller cooks as I had initially intended.
3) I would nix my probe ports, since I have never used them. I use them in my more "set it and forget it" smokers like the Cookshack and Webers, but the stick burner is a different kind of activity where I am constantly around the smoker, and I cook on it more "by feel" than precise readings.
4) Extra rack positions. I have the extended height on the cabinet. Three shelves is plenty, but I wish I had them put a set of shelf guides between each set of shelves to give me a couple more positioning options.
5) Raised firebox height. You will see that some SFs have a taller warmer with a shorter firebox (close to a 75/25 ratio like mine) while others have a ratio similar to 66/33 or 60/40 if I remember correctly. The later ratio seems to be with the thicker or insulated firebox options. Originally I liked the idea of having more warmer space and less firebox space, but sometimes I think it would be easier to add and arrange the splits in the firebox if I had a little more clearance.
6) I would have looked at getting solid shelves instead of the base shelves so when I pulled out the cooking racks to sauce meat the drippings would land on the shelf and not the landscape rocks.
I live in AZ so I can't really speak to the question of the insulated firebox.
