Depends. If you're lucking for just a plunder ally or longer term investment, the 50bil one.
If you're looking solely at stats - 5 10bil ones.
If you're looking for whatever will give you the highest turnover rate - that depends on the market, but remember that more people can afford 10 bil allies than 50 bil ones.
@ botan, you scout someone's spy to zero, then you scout once every few second until your enemies spy lost jump from zero to n number. There you have it, his/her regen time to the second. However, if the enemy use a crystal, the regen time reset to the moment the crystal was used.