From what I understand the quality of your life will be dependent on how well you are able to adapt to Dubai.
Unless you grew up in Phoenix, AZ, it is unlikely that you have ever lived in the circumstances you will find yourself in once relating to Dubai. But there are many people with actual experience on this forum more qualified to talk about that.
Considering you are still an F/O at your current job I wouldn't think twice and apply for Emirates. A quick upgrade is meaningless when flying for a Loco. Unless that is specifically your target. The upgrade in Flydubai will be the end of your career development in that company.
If you consider Emirates you might have 6 to 8 years in the right seat before you become admissible for an upgrade. Even if you are in your late twenties upon joining it still means you will be captain on a wide-body by mid-thirties.
Assuming you are able to survive in the desert for that long.
In general I would say the terms "quick upgrade" and "proper career" are not interchangeable.