Well actually... 50% con bonus means you pay only 66.66% of original costs. So actually its exactly the same since 100/1.5 (1.5 = 100% + 50% extra from ur econ bonus) = 66.66%
But there always is a certain balance that is optimal between 1) construction bonus and 2) econ or resources bonus. If think about it a bit u'll understand..
As you know the first research points are the most effective. Hereby i mean: The higher ur % is the more points are required to even get it higher.
Now with contruction there is a double effect present. Not only are the first % easier to get (similar to all other bonusses), but also they are alot more helpful. Since from 0-50% you save 33.33% and from 50-100% you save only 16.66%. So even if ur not a fan of getting a high construction bonus at start of round i'd advice everyone to get at least 10% construction bonus at least, it'll help you alot and its cheap.
So Forget the "VS" In the title. They go perfectly together, now its ur job as player to find the best balance, the balance you feel ok with.