if you take ignite or exhaust depndss so much on what lane you have in bot. What team they have. What summoners and champ played in your team at solo lanes. If your top laner and mid lanes takes ignite you should really consider exhaust. if you play vs swain top and vlad mid while your top takes tp and you mid laner say xerath take heal, then you should consider ignite.
if the enemy team have a say zed, trynd, yi, katarina, alkali, exhaust will do tons of works in team fights since dose champ are dependent on quick burst or/and resets with exhaust will deny.
if we only look at bot lane. you should take ignite vs heal supports like soraka, nami, sona etc.
recognize the strength of your bot lane. sway you play leona graves vs sona twitch you should take ignite for really strong kil persure.
if you play kogmaw nami vs lucian natiluse you should take exhaust to survive there all in.
then its a question of preference. if you prefer to be a aggressive support then take ignite if you feel comfortable with it. take exhaust if you prefer it.
i prefer ignite since i like the kill peruser and play very aggressive, its suite my play stayle and I'm know how to use it. I'm not as good with when to use exhaust. exhaust in general is harder summoner then ignite and can have hung impact or next to nothing depended on how you use it