Are you going to keep on playing there regularly if you win or are you looking to cashout soon if you win ? If you will keep playing anyway, then take the bonus.
It's hard to find a bonus and low vig combined. Usually you pay full vig when getting a bonus which makes the bonus worthless unless you successfully scalp it out on the 1st or 2nd bet.
You won't get a bonus at a place like 5Dimes very often. Even Phoenix puts you at full vig with a bonus. A bonus at Betjamaica would be great for baseball with their low vig on MLB sides.
A bonus at a shitty book ( like SBG, Oddsmaker, Sportsbook.com, Beted, etc.) should mean you are trying to scalp it out quickly.