If you can only fund one or the other, it would depend on some factors.
The Greek would be best if you like to bet higher amounts or like to bet lines when they first come out. There are also more prop bets, team totals, halftime lines, etc.
BetJamaica would be best if you bet smaller amounts, and if you figure on betting baseball a lot because they use dime lines.
Both have great service and are among the top books out there. You can also freely transfer funds between these two books as long as it is for over $500 each time.