You won't be needing steam injection for baking cookies. In any case, there are very few convections with steam injection, there are a lot of models that squirt a jet of water onto the fan, the water evaporates and turns to steam, but this cools down the oven considerably.
Electric ovens are probably a better bet. For one thing, most municipalities don't require electrics to be vented--so you don't need a hood or fire suppression. Electrics also have few or no moving parts, and a lot less maintainence than gas ones.
There's nothing better about convections for cookies. True, they take slightly less time to bake and use slightly less energy, but you have to open the doors and turn the trays around to get even baking. Every thime you open those huge doors you loose a lot of heat and cool down your oven as well as heat up your kitchen. True, combi ovens have fans that reverse every few minutes. but a combi oven--a 1/2 pan size will cost just as much or more than a decent 2 full pan deck oven. And a deck oven will bake better because you can control top and bottom heat.
So my vote--basded on cooking and baking commercially for 30 years, is an electric deck oven