I assume the fifty-fifty's are the same as double-up SnGs on Cake...where there is a $2+$0.20 buy-in and the top 5 out of 10 finishers win $4.
Playing these games, you can use winning percentage as an accurate stat, since there are only two options/prizes (win, $1.80 profit; loss, $2 loss of profit). In this scenario, you need a 55% winning percentage in order to break even. Anything below 55% (even if the winning percentage is above 50 percent) you're still taking a loss.
As for regular SnGs, where the prizes aren't even for everyone who is ITM (more prize money for 1st, less for lower finishers), the profitability comes down to not only how many times you finish ITM, but how many you win, and how many youare finishing last out of everyone ITM (and thus recieving the smallest prize).
If you find yourself winning a lot of SnGs, then regular SnGs might be for you. If you struggle to win them but always feel like you are right around the top half, then double up SnGs (or fifty-fifty's, as most people here apparantly call them) are for you. Remember, you need a 55% winning percentage to be profitable if there is a 10% house cut on the buyin!