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 in land size. Theo is thus a bit more difficult. The Theo destock takes longer (due to cheaper PM), but more efficient in $/NW at $222/NW instead of $262/NW, but it also means you "stock" a little bit less since you have to jump a bit earlier than a Rep. This isn't that big of a deal since a good Theo casher would want to have more land than a Rep anyway, so the slower PM destock is slightly mitigated by that.

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