Gas is better roasting meats and veg for your typical Sunday roast while electric is better for baking.
However if you go gas, it's imperative you get an oven with a fan like Richie's, otherwise the heat does not circulate properly and the rear of the oven will be hotter than the front causing uneven cooking if you have say, a large joint of meat that uses the entire shelf.
Another reason for them being uncommon is installation. Every house will have an electric point for an oven, but if your house has never had a gas oven, you may need to factor in the extra cost of a gas connection for it, which must obviously be done by a gas safe engineer and they don't come cheap nowadays. In fact, iirc, all gas ovens need to be installed by a gas safe engineer full stop. It might just be more expensive if you don't have somewhere to connect it to.