Which generation do you prefer to use everyday?
Honestly, making scenarios isn't going to make you any money so you may as well design them for your enjoyment first and foremost.
However you should also consider that with Gigamaps there is the temptation to max out on cities (to 255), which can still be well-spread out. The distances involved mean that the unit limit (2048) is more easily reached, because of the lower attrition rate.
Also, you should be considering the amount of event space you need as a primary factor, because with MGE you can squeeze in a extra 35-40% (in theory the dump is 32Kb compared to 16Kb, but that's just marketing).
There are other differences between the two, such as AI aggressiveness and civilopedia support; but MGE users can always play scenarios built with FW. FW users cannot play scenarios with over-sized maps or use MGE length event scripts.
My advice: go for the lowest common denominator (FW).