I have mates with both types. Both cook pizzas well but the brick ones hold their heat alot longer, and if you insulate between the outer & inner skin of a brick one, they can hold their heat for days.
We did a job at a power station a few years ago and one of my work mates took a piece of insulation from a turbine that runs at 1500 Celsius or something like that and he threw that in between the inner fire bricks and the outer brick skin when he was building it. He cooks pizzas on Friday night, then puts a roast in it on Saturday night, then his neighbours cook in it on Sunday night, without having to relight the fire after the Friday.
The convenience of just building a base and plonking one down is pretty handy though...
Yeah I've seen videos of people that have like three layers (Brick + insulation + mortar) and they hold temps for like three days!
There's obviously an added cost there if you want the convenience of having sustainable temps for more than a day.
I've been looking around and there's kits that you can buy from around $1800 up to around $3000 but obviously you get a range of inclusions and exclusions based on where you buy.
I'm leaning towards the Brick + insulation + mortar purely for the fact it should hold temps a lot longer.
but then again you have the cost saving factor for the pre fab moulds....