A Captain can be employed as an F/O.
An F/O cannot be employed as a Captain.
I would go for the ATR command, which will give you more options and more employability in the long run. In future job applications; airlines will see that you have passed a command course which makes you much more valuable to them than an F/O with unknown command ability, since they know you will pass another command course when the time comes. And since you are happy in your current job, aircraft type, and company, I would stay there for now.
(Some) big shiny jets are great, but not all airlines who fly them are. And you would have to give up your current roster and company, and probably have to fly very different roster patterns and Ts & Cs, which might be a backward step in terms of your work / life balance and overall happiness and quality of life.
If you go the jet F/O route now, it could be a long time before you get anywhere near a command course.