Rep has more potential. A Theo casher needs a bit more land than a Rep casher to have the same cash output per turn, so obviously a player that chooses Theo needs to take advantage of the faster start, and getting that land first, and staying ahead. Theo is thus a bit more difficult.

There is no particular cost effective % to get tech to. It depends on the tech price, and the number of turns left in the reset, and your current tech levels. You'll have to just do the math out. Also check out my post here: http://www.earthempires.com/forum/post/584264/perm