Both are very good and able to do the job and which is best depends on the map. Here is what I've found.
If you need to use ships to take the game further then cannons and grenadiers (along with pikes, knights and maces) also give access to galleons, frigates, privateers and ships of the line so go for them first. Privateers can really harass another continent and suppress their research rate. Cannons are so dominant if you have exclusive use of them that the follow up troops really don't have to be anything special maces and muskets easily kill whatever is collateralled down to 50%
If you're fighting mostly on land (and won't need ships for a while) and you want to draft then rifles with cuirassiers/ cavalry and trebuchets and spies is the way to go. This has the advantage of being really good for mid game espionage (jails, citadels and Nationhood civic = 100% EP) and the Taj Mahal can kick off a GA to boost production and give free civic switches. The espionage is great for seeing exactly what troops your targets have and where they are as well as what they're researching.
If you need to use ships to take the game further then cannons and grenadiers (along with pikes, knights and maces) also give access to galleons, frigates, privateers and ships of the line so go for them first. Privateers can really harass another continent and suppress their research rate. Cannons are so dominant if you have exclusive use of them that the follow up troops really don't have to be anything special maces and muskets easily kill whatever is collateralled down to 50%
If you're fighting mostly on land (and won't need ships for a while) and you want to draft then rifles with cuirassiers/ cavalry and trebuchets and spies is the way to go. This has the advantage of being really good for mid game espionage (jails, citadels and Nationhood civic = 100% EP) and the Taj Mahal can kick off a GA to boost production and give free civic switches. The espionage is great for seeing exactly what troops your targets have and where they are as well as what they're researching.