We have one , just look at oven capacity for both as a lot of them have two quite small ovens, difficult if you want to cook something like a turkey , Smeg were the best we found in terms of capacity and even this one is quite a lot smaller than our previous freestanding one
I can fit three racks in my lower oven or two in lower, one in upper. I have never had a problem with my oven's size. Lower oven is fan, too. The oven is Hotpoint but a model which is maybe 10 years old, I dunno it was put in by the previous owners.
Double ovens tend to be slightly smaller than a single, but not to an extent that it would affect most people. I've just produced Christmas lunch for 13 from my double oven, including a massive 16lb turkey, with no problems.
I couldn't do without the double oven; I often want to grill while something else is in the oven, or keep stuff warm.
IMO a double oven is better than a single, but I don't like built-under. Possibly if you are very gymnastic, or very short, you won't mind lifting scorching hot pans out, so near to the floor, or crouching down to look inside or to clean it.
I prefer one in a higher housing, with a big pan drawer underneath, and a cupboard above where you might keep crockery or other stuff which will not mind getting warm.
A double will need a dedicated electric circuit, not a plug and socket.